Hadīth is next important source of Islamic Law after the Qur'ān. It is widely observed that many Muslims (as well as non-Muslims) are unaware of how the ahādīth (pl.) are studied and classified into various categories. In this section we would briefly discuss the sciences of hadīth that leads to their classification into various grades of authenticity.

The Muslims are agreed that the Sunnah of the Prophet Muhammad (may Allah bless him and grant him peace) is the second of the two revealed fundamental sources of Islam, after the Glorious Qur'ān. The authentic Sunnah is contained within the vast body of Hadīth literature.

A hadīth (pl. ahādīth) is composed of two parts: the matn (text) and the isnād (chain of reporters). A text may seem to be logical and reasonable but it needs an authentic isnād with reliable reporters to be acceptable; cAbdullāh b. al-Mubārak (d. 181 AH), one of the illustrious teachers of Imām al-Bukhārī, said, "The isnād is part of the religion: had it not been for the isnād, whoever wished to would have said whatever he liked."

During the lifetime of the Prophet (may Allah bless him and grant him peace) and after his death, his Companions (Sahabah) used to refer to him directly, when quoting his sayings. The Successors (Tabicūn) followed suit; some of them used to quote the Prophet (may Allah bless him and grant him peace) through the Companions while others would omit the intermediate authority - such a hadīth was later known as Mursal. It was found that the missing link between the Successor and the Prophet (may Allah bless him and grant him peace) might be one person, i.e. a Companion, or two people, the extra person being an older Successor who heard the hadīth from the Companion. This is an example of how the need for the verification of each isnad arose; Imām Mālik (d. 179) said, "The first one to utilise the isnād was Ibn Shihab al-Zuhrī" (d. 124).