(a.), custom, customary law (also ʿāda , [q.v.]).
1. The status of custom in Islamic law
Custom was not one of the formal sources of law ( uṣūl , sing, aṣl ) in classical Islamic law. In practice, however, custom was frequently drawn on as a material source of law. Eventually, in the 16th century, it gained something close to formal recognition; but before that time attempts were made to incorporate custom in the law without granting it formal recognition, particularly (but not only) by Ḥanafī jurists.
Abū Yūsuf (d. 182/798), an early leader of the Ḥanafī school, was inclined to …