In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Shaikhul Islaam Ibn Taymiyyah [rahimahullaah] said:
The one who abandons the truth or some of it will be tried with falsehood. This is the case with ahlul bidah. You will not find anyone who has abandoned some of the Sunnah, which is to be affirmed and acted upon, except that he falls into bidah. You will not find a person of Bidah except that he has abandoned something from the Sunnah, as reported in the hadeeth, “A people do not bring about a Bidah, except after abandoning something similar to it of the Sunnah”. [Reported by Ahmad]
Allaah [The Exalted] said: [ فَنَسُواْ حَظًّ۬ا مِّمَّا ذُڪِّرُواْ بِهِۦ فَأَغۡرَيۡنَا بَيۡنَهُمُ ٱلۡعَدَاوَةَ وَٱلۡبَغۡضَآءَ – But they have abandoned a good part of the Message that was sent to them. So We planted amongst them enmity and hatred. [Surah Al-Maa’idah. Aayah 14] Therefore, after they abandoned a part of the message that was sent to them, they then held onto other it and thus enmity and hatred occurred between them.
Allaah [The Exalted] said: [ وَمَن يَعۡشُ عَن ذِكۡرِ ٱلرَّحۡمَـٰنِ نُقَيِّضۡ لَهُ ۥ شَيۡطَـٰنً۬ا فَهُوَ لَهُ ۥ قَرِينٌ۬ – And whosoever turns away (blinds himself) from the remembrance of the Most Beneficent (Allaah) (i.e. this Qur’aan and worship of Allaah), We appoint for him Shaitaan (Satan devil) to be a Qareen (an intimate companion) to him. [Surah Az-Zukhruf. Aayah 36]
فَإِمَّا يَأۡتِيَنَّڪُم مِّنِّى هُدً۬ى فَمَنِ ٱتَّبَعَ هُدَاىَ فَلَا يَضِلُّ وَلَا يَشۡقَىٰ
وَمَنۡ أَعۡرَضَ عَن ذِڪۡرِى فَإِنَّ لَهُ ۥ مَعِيشَةً۬ ضَنكً۬ا وَنَحۡشُرُهُ ۥ يَوۡمَ ٱلۡقِيَـٰمَةِ أَعۡمَىٰ
Then if there comes to you guidance from Me, then whoever follows My Guidance shall neither go astray, nor fall into distress and misery. But whosoever turns away from My Reminder (i.e. neither believes in this Qur’aan nor acts on its orders, etc.) verily, for him is a life of hardship, and We shall raise him up blind on the Day of Resurrection. 20:123-124
And Allaah [The Exalted] said: [ ٱتَّبِعُواْ مَآ أُنزِلَ إِلَيۡكُم مِّن رَّبِّكُمۡ وَلَا تَتَّبِعُواْ مِن دُونِهِۦۤ أَوۡلِيَآءَۗ قَلِيلاً۬ مَّا تَذَكَّرُونَ – Follow what has been sent down unto you from your Lord (the Qur’aan and Prophet Muhammad’s Sunnah), and follow not any Auliyaa’ (protectors and helpers, etc. who order you to associate partners in worship with Allaah), besides Him (Allaah). Little do you remember! [Surah Al-A’raaf. Aayah 3]
So, they have been commanded to follow what has been sent down, and prohibited from that which is in opposition to it, which is to follow protectors and helpers besides Him (Allaah). So whoever does not follow one of them will follow the other. This is why Allaah [The Exalted] said: [ وَمَن يُشَاقِقِ ٱلرَّسُولَ مِنۢ بَعۡدِ مَا تَبَيَّنَ لَهُ ٱلۡهُدَىٰ وَيَتَّبِعۡ غَيۡرَ سَبِيلِ ٱلۡمُؤۡمِنِينَ نُوَلِّهِۦ مَا تَوَلَّىٰ وَنُصۡلِهِۦ جَهَنَّمَۖ وَسَآءَتۡ مَصِيرًا – And whoever contradicts and opposes the Messenger (Muhammad) after the right path has been shown clearly to him, and follows other than the believers’ way [i.e. the Sahaabah]; we shall keep him in the path he has chosen, and burn him in Hell – what an evil destination. [Surah An-Nisaa. Aayah 115]
The Ulamaa say, “The one who does not follow their path [i.e. the path of the Sahaabah] will follow other than their path”; so they utilise this as proof that it is obligatory to follow their path [i.e. that of the Sahaabah]. And it is not for anyone to leave that which they are agreed upon.
[Source: An Excerpt from ‘Ad’waa Min Fataawaa 2/864’. By Shaikh Saaleh Al-Fawzaan (hafidhullaah). Slightly paraphrased]