In many Ayahs (Qur'anic verses), Allah (Glorified and Exalted be He) indicates the obligation of following this Book (Qur'an), adherence to it and observation of its limits.
Follow what has been sent down unto you from your Lord (the Qur'ân and Prophet Muhammad's Sunnah), and follow not any Auliyâ' (protectors and helpers who order you to associate partners in worship with Allâh), besides Him (Allâh). Little do you remember!
And this is a blessed Book (the Qur'ân) which We have sent down, so follow it and fear Allâh (i.e. do not disobey His Orders), that you may receive mercy (i.e. be saved from the torment of Hell).
Indeed, there has come to you from Allâh a light (Prophet Muhammad ﺻﻠﻰ ﺍﷲ ﻋﻠﻴﻪ ﻭﺳﻠﻢ) and a plain Book (this Qur'ân). Wherewith Allâh guides all those who seek His Good Pleasure to ways of peace, and He brings them out of darkness by His Will unto light and guides them to the Straight Way (Islâmic Monotheism).
I am leaving you that which if you adhere to you will not go astray: the Book of Allah.
I am leaving among you two weighty things, the first of which is the Book of Allah in which there is guidance and light, so hold fast to the Book of Allah and adhere to it.