Quran the book of religious custom :

Quran is mentioned in Nahj-ol-balaqe as the best source and most trustful reference to recognize (Islam) religion in cognition of God, prophetic mission of prophets, divine books, resurrection, the other world, descriptions of heaven and hell, religious lawful and unlawful, the behaviors of different people in society, etc. Quran is the criterion of accuracy and inaccuracy of religious thoughts, beliefs and opinions in territory of religion. Even Hadiths, if are in disagreement with the very text of words of Allah (Quran), they are unworthy.

In oration 86, Imam Ali (AS) stated about the religion of God, Prophet (PH) and the book of Allah (Quran):

((Allah has sent his book to you which is clarification of everything. And made his prophet to live among you for a while and have perfected his religion, as his favorite is clarified in his book (Quran). And by the tongue of his prophet, have reached what is good and what is bad to him, orders and prohibitions to you and have offered a conclusive argument.))

Also he stated in the first oration:

((Prophet (PH) died while he left a clear way among you; which means the book of your God (Quran) that explains the religious lawful and unlawful (revealed) from Allah, religious duties and recommended deeds, abolishing and abolished (Naskh and Mansoukh), general and specific (Aam and Khas) beside eye-openers, parables, clear and ambiguous (Mohkam and Motashabeh), etc.))

Through the gnostic knowledge of Quran, also, people reach religious unity and if they do not put Quran as their reference, the way is made for innovations and falsifies. Imam Ali (AS) have pointed to desires to speak about the emersion of seditions and innovatory religious rules which are against the book of Allah (Quran): (یخالف فیه کتاب الله) (the book of Allah is been disagreed in it) (oration 50) if Quran is the central object and standard, there would remain no way for void thoughts and innovatory religious rules.

Quran the book of religious custom

Someone asked Imam Ali (AS) about the attributes of Allah. Imam (AS) stated some significant points about cognition of God, about the religion and about the school (of thought) in oration 91, which is famous as ghost oration, and the later part is a piece of it:

((O the asker! Look well at what Quran mentions of attributes of Allah, follow them, accept them and take clarity from the light of Quran. But let the knowledge of what Satan forces you to do, which there are not any trace of them in Quran or in the tradition of Prophet (PH) or leaders of guidance.))

In the very same oration and also oration 183 also he has a speech with same content where he means Quran is the proof of God on servants. Allah has completed his light and has perfected his religion through this book (Quran). So the criterion of the lawful and unlawful of religion of Allah, is the holy words of Allah (Quran).

Imam Ali (AS) in letter 69, which has been written to Hareth hamedani, advise as so:

((Grasp the string of Quran, seek benevolence of it, suppose the lawful of it as permitted and unlawful of it as unpermitted.))

When Imam (AS), introduces the holy Quran as the base and foundation of Islam (in oration 198), identifies it as the source and the root of faith (in oration 198) and knows it as the guidance to reach God, so in cognition of religious custom, religious lawful and unlawful, principles of faith (the doctrine of religion) and elements of the religion we should refer to Quran and also to the ones who are scholars of Quran and familiar with secrets and knowledge of revelation, which Ahl-e-bayt (AS) (the family of Prophet Mohammad PH; including Imam Ali AS, Lady Fateme SA, Imam Hasan (AS), Imam Hussain AS and their sons) are at the highest level of them.

Imam Ali (AS) in oration 147 has stated when reporting the manner of Prophet of Islam (PH):

((Allah has chosen Prophet Mohammad (PH) rightfully in order to change the servants from paganism to theism and move them from obeying Satan to obey God; through the Quran which has made it clarifier and firm in order to people, who did not know their God, know him (Allah) and have faith after the period of denial and infidelity.))