he basic sources of legal evidence are the Book, that is, the Quran, and the Prophetic traditions, which clarify the Quran. At the time of the Prophet the laws were received directly from him. He possessed the Quranic revelation, and he explained it directly. No transmission, speculation or analogical reasoning was necessary. After the Prophet’s death direct explanation was no longer possible. The Quran was preserved through a general and continuous transmission. As for the Prophetic tradition, the men around Muhammad all agree that it was necessary to act in accordance with whatever of it has reached us...through a sound report that can be trusted to be truthful...
Then general consensus took its place next to them. The men around Muhammad agreed to disapprove of those who held opinions different from theirs. They would not have done that
without some basis for doing so, because people like the men around Muhammad do not agree upon something without a valid reason. In addition, the evidence attests the infallibility of the whole group. Thus, general consensus became a valid proof in legal matters.
Then we looked into the methods according to which the men around Muhammad and the early generations made their deductions from the Quran and the Prophetic traditions. It was found that they compared similar cases and drew conclusions from the analogy...Many of the things that happened after the Prophet are not included in the established texts. Therefore they compared and combined them with the established indications that are found in the texts, according to certain rules that governed their combinations. This assured the soundness of their comparison of two similar cases, so that it could be assumed that one and the same divine law covered both cases. This became another kind of legal evidence, because the early Muslims all agreed upon it.