In The Name of Allah, The Most Merciful, The Bestower of Mercy.

Unity

Imam as-Sadi [may Allah have mercy upon him] said:

One of the greatest Islamic principles is that Muslims must adhere to their religion, fulfil servitude to Allah and Allah’s rights, fulfil people’s rights, be in agreement and unity (footnote a), strive to adhere to what will bring about harmony and love, and remove animosity and grudges.

Allah [The Exalted] said: [ إنما المؤمنون إخوة – Verily the believers are brothers in Islamic faith]. [Surah Al-Hujuraat. Aayah 10]

Allah said: [واذكروا نعمة الله عليكم إذ كنتم أعداء فألف بين قلوبكم فأصبحتم بنعمته إخوانا – And hold fast, all of you together, to the Rope of Allah (i.e. this Qur’an), and be not divided among yourselves, and remember Allah’s Favour on you, for you were enemies one to another but He joined your hearts together, so that, by His Grace, you became brethren (in Islamic Faith)]. [Surah Al Imran. Ayah 103]

Allah said: [فاتقوا الله وأصلحوا ذات بينكم وأطيعوا الله ورسوله إن كنتم مؤمنين – So fear Allah and adjust all matters of difference among you, and obey Allah and His Messenger (Muhammad ), if you are believers]. [Surah Al-Anfaal. Ayah 1]

Allah said: [ولا تكونوا كالذين تفرقوا واختلفوا من بعد ما جاءهم البينات – And be not as those who divided and differed among themselves after the clear proofs had come to them]. [Surah Aal Imran. Aayah 105]

Allah said: [واعتصموا بحبل الله جميعا – And hold fast, all of you together, to the Rope of Allah (i.e. this Qur’an and the Sunnah)]. [Surah Al Imran. Ayah 103]

There are several other texts that illustrate this important principle, which elevates Muslims to the pinnacle of human perfection and establishes their affairs on an upright foundation.

Allah said:
يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا لَقِيتُمْ فِئَةً فَاثْبُتُوا وَاذْكُرُوا اللَّهَ كَثِيرًا لَّعَلَّكُمْ تُفْلِحُونَ
وَأَطِيعُوا اللَّهَ وَرَسُولَهُ وَلَا تَنَازَعُوا فَتَفْشَلُوا وَتَذْهَبَ رِيحُكُمْ ۖ وَاصْبِرُوا ۚ إِنَّ اللَّهَ مَعَ الصَّابِرِينَ
وَلَا تَكُونُوا كَالَّذِينَ خَرَجُوا مِن دِيَارِهِم بَطَرًا وَرِئَاءَ النَّاسِ وَيَصُدُّونَ عَن سَبِيلِ اللَّهِ ۚ وَاللَّهُ بِمَا يَعْمَلُونَ

O you who believe! When you meet (an enemy) force, take a firm stand against them and remember the Name of Allah much (both with tongue and mind), so that you may be successful. And obey Allah and His Messenger, and do not dispute (with one another) lest you lose courage and your strength depart, and be patient. Surely, Allah is with those who are As-Sabirin (the patient ones, etc.). And be not like those who come out of their homes boastfully and to be seen of men, and hinder (men) from the Path of Allah. and Allah is Muhitun (encircling and thoroughly comprehending) all that they do. [Surah Al-Anfaal. Ayaat 45-47]

Allah commanded them to follow Him and His Messenger, which encompasses all of the religion; forbade them from disagreeing since that would cause discord and animosity, which would weaken faith. He commanded them to remember Him a lot in every matter and to exercise patience, on which every matter depends; commanded them to be sincere and truthful, and forbid what is contrary to it, such as Riyaa (see footnote b), bragging, pride, evil intentions, and the desire to mislead people. (footnote c) (1)

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Footnote a: Listen to the lesson here: https://www.salafisounds.com/unity-and-a-dispraise-of-splitting-by-abu-khadeejah-abdul-wahid/

Footnote b: Listen to the lesson here: https://www.salafisounds.com/ar-riya-doing-deeds-of-worship-to-show-onlookers-kitab-at-tawhid-by-abu-khadeejah/

Footnote c: Iyad Ibn Himar [may Allah be pleased with him] reported that Allah’s Messenger [peace and blessings of Allah be upon him] said, “Verily, Allah has revealed to me that you must be humble towards one another so that no one oppresses another or boasts to another”. [Ṣaḥeeh Muslim 2865]

Ref 1: An Excerpt from Ad-Deenus Saheeh Yahillu Jamee al-Mashaakil (The right religion is the answer to every problem)-Page 14. slightly paraphrased