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The Hadis and Sunnah either:

1) Explain a verse of the Quran or add something to its commandment. In this case, it will have a secondary position to the Quran as it will be a based on the Quran itself.

2) Lay down new rules in matters about which the Quran is silent. In this case, it will stand on its own force and form the basis for legislation. The Scholars are of the view that the Sunnah can be the sole source of Islamic law in those matters that have not been dealt with in the Quran.

The Quran, being the word of Allah, contains the principles of religion and treats major issues, and often deals with subjects in brief terms, The Hadis and Sunnah of the Prophet PBUH contain its details and particulars. The Quran says:

… And we have sent down unto thee (also) the Message: that thou mayest explain clearly to men what is sent for them….” (16:44)
While the Quran’ gives the Muslims a primary rule of life, there are many matters where guidance for practical living is necessary, but about which the Quran says nothing. In such cases the obvious thing was to follow the Sunnah of the Prophet PBUH, For who else could a better guide than the Prophet himself who provided an oral and practical interpretation and commentary as an aid to the correct understanding of the meaning of the Quran. The structure of the Shariah is thus, based on the Quran and the words and actions of the Holy Prophet PBUH In the Quran, we are commanded to pray and pay Zakat, but there is no explanation of prayers or the number of raka’ ats in each prayer. Neither is there an explanation given as to the amount on which the duty of Zakat applies or the percentage which has to be paid. All these details were learnt either from the Prophet’ deeds or words or from both. The Holy Quran commands that we should be good to our neighbours a number of Hadis further stress this duty. The same applies to many other situations; hence the vital connection between the Quran and the practice and sayings of the Prophet PBUH. 
Many verses of the Holy Quran deal with questions of law but not all the injunctions of the Shariah are clearly stated in it. There are many statements which needed further explanations before they could become guides for human actions. This explanation and clarification was provided by the Holy Prophet PBUH.
When the Islamic common wealth was founded in Madinah in 622 A.D., the Prophet PBUH was not only the spiritual leader but also the supreme judge of the Islamic community. Cases would be referred by his followers to the Prophet PBUH for his judgement. The underlying principle applied was the word of Allah from the Quran; but if the Quran was silent, or needed interpretation, the Prophet PBUH as the messenger of Allah would be the authority for reaching a decision.
The Holy Quran itself commands that the Prophet’s PBUH teachings should be obeyed. Whenever the Quran commands its followers to obey Allah, it also commands them to obey the Prophet PBUH. It declares that obedience to the Prophet PBUH is obedience to Allah Himself. The Quran says: “He who obeys the Messenger obeys Allah.” (4: 80)
The Quran frequently commands:
.”Obey Allah and obey the Messenger.” (5:92, 64:12)

The Prophet’ s PBUH Sunnah and Hadis along with the Quran, are the fountain- head of all Islamic law and thought.