In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
We Must Restrain The Tongue
The Messenger of Allaah (sallalaahu-alayhi-wasallam) said: When the son of Adam wakes up in the morning, all the limbs humble themselves before the tongue and say: ”Fear Allaah for our sake, (for) we are with you; if you are upright, we will be upright; and if you are crooked, we will become crooked. 
The Messenger (sallal-laahu-alayhi-wasallam) also said: Whoever can guarantee (the chastity of) what is between his two jaw bones and what is between his two legs (i.e. his mouth, his tongue and his private parts), I guarantee Paradise for him. 
Imaam Abdul-Azeez Bin Baaz (rahimahullaah) said: This requires two affairs; [a] A lot of remembrance (of Allaah); [b] keeping away from gatherings of heedlessness and doubts, and (keeping away from) that which will result in fornication and nonsense speech. 
We Must Enjoin Good And Forbid Evil
Allaah [The Most High] said:
لَّا خَيۡرَ فِى ڪَثِيرٍ۬ مِّن نَّجۡوَٮٰهُمۡ إِلَّا مَنۡ أَمَرَ بِصَدَقَةٍ أَوۡ مَعۡرُوفٍ أَوۡ إِصۡلَـٰحِۭ بَيۡنَ ٱلنَّاسِۚ وَمَن يَفۡعَلۡ ذَٲلِكَ ٱبۡتِغَآءَ مَرۡضَاتِ ٱللَّهِ فَسَوۡفَ نُؤۡتِيهِ أَجۡرًا عَظِيمً۬ا
There is no good in most of their secret talks save (in) him who orders Sadaqah (charity in Allaah’s Cause), or Ma’roof (Islamic Monotheism and all the good and righteous deeds which Allaah has ordained), or conciliation between mankind. [Surah An-Nisaa. Aayah 114]
Meaning: There is no good in most of their private conversations; therefore, if there is no good in it, then there will be no benefit in it, such as excessive permissible speech, speech that is evil and completely harmful and all types of unlawful speech. Then Allaah [Glorified be He and free is He from all imperfections] made an exception, saying: [إِلَّا مَنۡ أَمَرَ بِصَدَقَةٍ – Save (in) him who orders Sadaqah (charity in Allaah’s Cause)]- Meaning: through wealth, knowledge or any means of benefit. And included in this are the lesser acts of worship, such as Tasbeeh [the utterance of Subhaanallaah] and Tahmeed [the utterance of Alhamdulil-laah] and what is similar, as the Prophet [sallal-laahu-alayhi-wasallam] said: ‘’Indeed, every Tasbeeh (utterance of Subhaanallaah) is sadaqah, and every Takbeer (utterance of Allaahu Akbar) is Sadaqah, and every Tahleel (utterance of Laa-ilaaha-ilal-lah) is Sadaqah, and Enjoining Good is Sadaqah, and Forbidding Evil is Sadaqah, and even when one of you has sexual intercourse [with his wife], there is Sadaqah in that’’. [Reported by Muslim]
Then Allaah [Glorified be He and free is He from all imperfections] said: [أَوۡ مَعۡرُوفٍ – Or (one) who orders Maruf] – Meaning: perfection in worship and obedience, every good in the Islamic legislation and everything whose goodness is known by way of sound intellect [i.e. not in opposition to the Islamic legislation]. And whenever ‘Enjoining Good’ is mentioned and ‘Forbidding Evil’ is not mentioned alongside it, ‘Forbidding Evil’ is also included. That is because to abandon what is prohibited is part of good deeds and good deeds cannot be complete except by abandoning evil. And in relation to [dealing] with companions [or associates], Ma’roof is explained to mean commanding [good] and Munkar means to abandon what is prohibited. [Ref 4]
Shaikh Abdur Razzaaq Al-Badr [hafidhahullaah] quotes from Imaam Ibnul Qayyim [rahimahullaah] who stated: Disobedience [can] appear agreeable to a human being, such as Gheebah [backbiting], Nameemah [tale carrying], [blameworthy] argumentation, self-praise – whether uttered blatantly or indirectly, verbally abusing the one you hate, praising the one you love and what is similar to that. The intense urge [i.e. the urge to engage in these deeds] agrees with [the soul’s desire], then it becomes easy for the tongue to move [i.e. speak] and thus one’s patience weakens; so due to this the Messenger [sallal laahu-alayhi wasallam] said to Mu’aadh [radiyallaahu-anhu], “Restrain your tongue”, so Mu’aad said, “O Messenger of Allaah! Will we be held accountable for what we say?” He [sallal laahu alayhi wasallam] said, “May your mother be bereaved of you, O Mu’aadh! Is there anything that topples people on their faces – or he said on their noses into the fire- other than the harvest of their tongues?!”
[This urge occurs], especially when sinning through the tongue is habitual because it entices a person towards impatience [i.e. he does not exercise patience in order to restrain his tongue]. This is why you find that a man performs the night prayer [i.e. Tahajjud, Qiyaamul Layl] and fasts during the day; as well as refrain from reclining on a pillow made of silk -out of fearing that he might enter into a doubtful affair that will affect him in the afterlife- but he backbites, spread tales and violates the honour of the people. [Ref 5]
Shaikhul Islaam Ibn Taymiyyah [rahimahullaah] said:
Amongst the people there is one who backbites to show his agreement to those whom he sits with- his companions and family- even though he knows that the person being backbitten is innocent of what they say or some of what they say; but he sees that if he rebukes them, the gathering will cease, the people in the gathering will become annoyed and turn away from him; therefore he sees that to be in agreement with them is good treatment and good companionship. And indeed when they get angry, he might also get angry due to their anger. [i.e. to show his agreement].
And amongst the people there is one who backbites in various ways- sometimes in the guise of religious affairs and rectification, whilst saying: ‘’I do not usually mention anyone except with good and neither do I like backbiting nor lying; rather I only inform you about his state of affairs’’, and then he says: ‘’By Allaah! Indeed, such and such person is a Miskeen or a good man, however there is such and such about him’’; or he might say: ‘’Leave him alone! May Allaah forgive us and him’’, however his intention is to belittle the person and elevate himself. So, he initiates backbiting in the guise of rectification and religious affairs. Indeed, we have seen – a lot – this [type of behaviour] from them and what is similar.
And amongst them there is one who raises the status of others out of show off and to raise his own status as well, saying: ‘’If only I supplicate for such and such person in my prayer due to such and such news that has reached me about him’’, in order to raise his own status and make this statement in the presence of someone who believes that he truly wants to supplicate for that person; or he says, ‘’Such and such person is simple minded and has little understanding’’, whilst intending to praise himself, affirm his own understanding and that he is more virtuous than that person.
And amongst them is one who backbites due to envy, so he gathers between two repugnant traits – backbiting and envy. And if a person is praised, he seeks to stop such praise by mentioning the shortcomings of that person in the guise of religious advice and rectification, or through wicked speech and defamation to nullify that praise.
And amongst them is one who initiates backbiting in the guise of mockery and jest in order to make others laugh- mocking and belittling people. And amongst them is one who initiates backbiting in the guise of amazement, saying: ‘’I am amazed by such and such person as to why he did not do such and such, and why he falls into such and such, or why he did such and such’’; so he mentions the name of the person in the guise of amazement.
And amongst them is one who pretends to be grieved, saying: ‘’Such and such person is Miskeen. What has happened to him grieves me’’, so the listener thinks that he is grieved and feels sorry, whilst his heart is happy due to the [misfortune] that person finds himself in. And if he was able, he would have increased that [misfortune] and maybe even mentions it to the enemies of the person. This [behaviour] and other than it is one of the greatest diseases of the heart. And amongst them is one who initiates backbiting in the guise of anger and forbidding evil, so he manifests beautified speech whilst intending other than what he manifests. And Allaah’s Aid is sought! [Ref 6]
Situations in which backbiting is permissible due to an overriding sharee’ah benefit
Backbiting is permissible in several situations when the need arises: When enquiring about the affair of someone you want to marry or before establishing a business partnership with someone. When complaining to the ruler about the oppressive behaviour of someone and seeking to stop the oppression. In these situations, there is no harm- due to an overriding benefit – in mentioning about a person in his absence that which he hates to hear. Some of the scholars mention the situations in which backbiting is permissible: [a] Complaint given by one who is wronged; [b] When describing a person; [c] When giving a warning against open evil [i.e. bidah committed by certain people]; [d]: Warning against a person who openly commits his sins. [e] When seeking a fatwa and [f] when seeking the help to stop evil. [Ref 7]
Flattery: Imaam Ibnul Qayyim [rahimahullaah] said: The one who flatters himself or others cannot smell the fragrance of truthfulness. [Ref 8]
Mockery- The Jahl of a Scornful Person
Allaah [The Most High] said:
وَإِذْ قَالَ مُوسَىٰ لِقَوْمِهِ إِنَّ اللَّهَ يَأْمُرُكُمْ أَن تَذْبَحُوا بَقَرَةً ۖ قَالُوا أَتَتَّخِذُنَا هُزُوًا ۖ قَالَ أَعُوذُ بِاللَّهِ أَنْ أَكُونَ مِنَ الْجَاهِلِينَ
And (remember) when Musa (Moses) said to his people: “Verily, Allah commands you that you slaughter a cow.” They said, “Do you make fun of us?” He said, “I take Allah’s Refuge from being among Al-Jahilun (the ignorants or the foolish). [2:67]
A Jaahil [ignorant one] is that one who speaks with a speech that contains no benefit and he mocks at the people. As for a sensible person, he sees that one of the most blameworthy traits -which is abhorred by [sound] religion and intellect- is to mock at someone who is a human being like himself. And if he has been favoured over another person, then this favour necessitates that he thanks Allaah and shows mercy to Allaah’s servants. Therefore, when Musa [alayhis-salaam] said this to them [i.e. I take Allaah’s Refuge from being among the ignorant], they knew that it [i.e. what he commanded them] was truth, so they said: [ادْعُ لَنَا رَبَّكَ يُبَيِّن لَّنَا مَا هِيَ -call upon your Lord for us that He may make plain to us what it is!(2:68)] [Ref 9]
The Difference Between Having a High Aspiration And Excessive Feeling of Self-importance
Shaikh Uthaymeen [rahimahullaah] said: To have a high aspiration [or a high ambition in the path of seeking knowledge] is that a person preserves his time, knows how to utilise it and does not waste time instead of utilising it in something beneficial. And when [another] person comes to him and he sees that sitting with such person will distract [or divert him from what is beneficial, befitting etc], he knows how to act [behave or react]. As for pride [or having an excessive sense of one’s own importance], it is about the behaviour of that one who belittles others and he does not consider people to be anything else other than frogs. And it may be that he would turn his face away whilst speaking to the people, just as the author stated that the difference between having high aspirations and pride is like the difference between the sky with [clouds] that give rain, again and again and the earth which splits [with the growth of trees and plants]. [Sharh Hilyati Taalibil Ilm. Page 163]
Concealment and Misrepresentation During Dispute And Disagreement
Narrated Umm Salamah [radiyallaahu-anhaa]: The Messenger of Allaah [sallal-laahu-alayhi-wasallam] said: ‘’Indeed, I am only a human being and you people (i.e. disputants) come to me with your disputes. And it may be that one of you can present his case more eloquently than the other and I consider him truthful, and judge in his favour. So if I ever judge and give the right of a brother to his brother, then it is a piece of hell-fire and let him not take it.” [Saheeh Al-Bukhaari -Kitaab Al-Ahkaam (Book of Judgements): Chapter 29: Hadeeth Number: 7181]
Some Benefits Derived From This Hadeeth:
[a] The Ummah [i.e. the scholars and judges] have been commissioned to judge based on what is apparent; but the mere judgement of a judge cannot prohibit the lawful and allow the unlawful [i.e. because it cannot be taken if proven erroneous].
[b] This Hadeeth contains a refutation against those who say that the Messengers possess knowledge of the unseen.
[c] This hadeeth also shows that speech can be seen to be true based on what is apparent, but it is truly falsehood with regards to what is hidden within it.
[d] Also this hadeeth shows that the one who receives a judgement in his favour is more aware than every other person as to whether he is entitled to it or whether he is a falsifier. So, he takes it if he is entitled to it or leaves it if he a falsifier, because in reality a judgment cannot change an affair from what it was in origin [i.e. the original truth in the affair before its distortion or concealment].
[e] Also this hadeeth shows the sinfulness of the one who argues based on falsehood until he receives what he wants publicly, whilst he is upon falsehood.
[f] Also in this hadeeth is proof to show that a scholar can make a mistake and it is a refutation against those who say that every Mujtahid is correct.
[g] Also this hadeeth shows us that the Mujtahid is forgiven (when he makes a mistake). [Ref 10]
False Accusation: The Prophet [sallal laahu alayhi wasallam] said: Whoever accuses a believer of something that is not true, Allaah will make him dwell in flowing pus on the Day of Judgement. [Saheeh Al-Jaami 6169]
 [Reported by Tirmidhee Number 2407: Chapter: What has been reported concerning preservation of the tongue] [Hadeeth Delared Hasan By Imaam Albaani]
 [Saheeh al-bukhaari, Book of Ar-Riqaaq, Vol 8, page: 263. Hadeeth Number: 6474]
 Al-Hulalul Ibreeziyyah Min At-Taliqaat Al-Baaziyyah Alaa Saheeh al-Bukhari. Vol 4. Page:244. Footnote:2]
[Ref4: An Excerpt from ‘Tafseer Al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan’. Slightly paraphrased]
[Ref 5: Quoted by Shaikh Abdur Razzaaq Al-Badr (hafidhahullaah) in Sharh Ar-Risaalah Al-Mughniyah Fis Sukoot Wa Luzoomil Buyoot. Lesson 1]
[Ref 6: An Excerpt from ‘Majmool Al-Fataawaa 28/236-238. slightly paraphrased]
[Ref 7: Fataawa Lajnah Ad-Daa’imah 20/26]. slightly paraphrased]
[Ref 8: Al-Fawaa’id. 185]
[Ref 9: An Excerpt from Tayseer al-Kareem ar-Rahmaan Fee Tafseer Kalaam al-Mannaan). slightly paraphrased]
[Ref 10: An Excerpt From ‘Saheeh Al-Bukhaari – Kitaab Al-Ahkaam (Book of Judgements) with Fathul Baari. Chapter 29: Hadeeth Number: 7181]