The heritage of Islam, particularly its jurisprudence, has four sources - two founded on historical records going back to the time of Muhammad and two on the development of the science of interpretation in the early centuries of Islam.

The Qur'an has always been regarded as the primary legal source of Islam but, when it was found necessary to look elsewhere for guidance, the early jurists of Islam turned to the Hadith. Only when both of these failed to provide the authority sought did they resort to ijtihad (interpretation) until they reached ijma (consensus). In the very early days of Islam Muslim authorities tended to rely on their own opinions to establish their interpretation of what a prescribed law should be for any given situation not founded on the Qur'an, a practice known as ra'y .

The great jurist ash-Shafi'i, however, preferred to rely solely on traditions from the prophet and thereafter on the method known as qiyas (analogy) where interpretations were to be derived from comparisons with relative subjects dealt with in the Qur'an or the traditions. Once Shafi'i's school of law was fully established together with the other great schools founded by Ahmad ibn Hanbal , Abu Hanifa and Malik, the "door" of ijtihad was closed and it was considered that ijma had been reached on all necessary points of law (though the schools differ in many matters to this day but mostly on minor points of interpretation). Accordingly, Islamic jurisprudence has for centuries known no real development and is based fundamentally on the four sources mentioned.

Reference

• http://www.sunnah.org/fiqh/usul/Default.htm
• http://www.ymofmd.com/books/uaf/default.htm
• http://www.usc.edu/dept/MSA/law/alalwani_usulalfiqh/
• http://onlineislamicstore.com/b4908.html