(from Arabic sanad, “support”), in Islam, a list of authorities who have transmitted a report (ḥadīth) of a statement, action, or approbation of Muhammad, one of his Companions (Ṣaḥābah), or of a later authority (tabīʿ); its reliability determines the validity of a ḥadīth. The isnād precedes the actual text (matn) and takes the form, “It has been related to me by A on the authority of B on the authority of C on the authority of D (usually a Companion of the Prophet) that Muhammad said.…”

During Muhammad's lifetime and after his death, ḥadīths were usually quoted by his Companions and contemporaries and were not prefaced by isnāds; only after a generation or two (c. AD 700) did the isnād appear to enhance the weight of its text. In the 2nd century AH (after AD 720), when the example of the Prophet as embodied in ḥadīth—rather than local custom as developed in Muslim communities—was established as the norm (sunnah) for an Islamic way of life, a wholesale creation of ḥadīths, all “substantiated” by elaborate isnāds, resulted. Since ḥadīths were the basis of virtually all Islamic scholarship, especially Qurʾānic exegesis (tafsīr) and legal theory (fiqh), Muslim scholars had to determine scientifically which of them were authentic. This was done by a careful scrutiny of the isnāds, rating each ḥadīth according to the completeness of its chain of transmitters and the reliability and orthodoxy of its authorities (see ilm al-ḥadīthʿ).

Early compilations of the most reliable ḥadīths (known as musnads) were even arranged by isnād; that is, classified according to the Companion of Muhammad to whom they were attributed. Most notable of these was the musnad of Aḥmad ibn Ḥanbal (d. 855), incorporating about 29,000 traditions. Musnads proved difficult to use efficiently, however, and later compilations, known as muṣannaf, grouped ḥadīths according to subject matter.