The Quran

فَلا وَرَبِّكَ لا يُؤمِنونَ حَتّىٰ يُحَكِّموكَ فيما شَجَرَ بَينَهُم ثُمَّ لا يَجِدوا في أَنفُسِهِم حَرَجًا مِمّا قَضَيتَ وَيُسَلِّموا تَسليمًا

Muhammad Asad

But nay, by thy Sustainer! They do not [really] believe unless they make thee [O Prophet] a judge of all on which they disagree among themselves, and then find in their hearts no bar to an acceptance of thy decision and give themselves up [to it] in utter self-surrender.¹

This verse lays down in an unequivocal manner the obligation of every Muslim to submit to the ordinances, which the Prophet, under divine inspiration, promulgated with a view to exemplifying the message of the Qur'an and enabling the believers to apply it to actual situations. These ordinances constitute what is described as the sunnah (lit., "way") of the Prophet Muhammad, and have (whenever they are authenticated beyond any possibility of doubt) full legal force side by side with the Qur'an: see verse 80 of this surah.