The Muslims are agreed that the Sunnah - the way Prophet Muhammad, the Messenger of Allah, lived his life (may Allah bless him and grant him peace) - is the second of the two revealed fundamental sources of Islam, after the Glorious Qur'an. The authentic Sunnah is contained within the vast body of Hadith literature.
A hadith (pl. ahadith) is composed of two parts: the matn (text) and the isnad (chain of reporters). A text may seem to be logical and reasonable but it needs an authentic isnad with reliable reporters to be acceptable; 'Abdullah b. al-Mubarak (d. 181 AH), one of the illustrious teachers of Imam al-Bukhari, said, "The isnad is part of the religion: had it not been for the isnad, 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 (Tabi'un) 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 hadith 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 hadith from the Companion. This is an example of how the need for the verification of each isnad arose; Imam Malik (d. 179) said, "The first one to utilise the isnad was Ibn Shihab al- Zuhri" (d. 124).
The other more important reason was the deliberate fabrication of ahadith by various sects which appeared amongst the Muslims, in order to support their views (see later, under discussion of maudu' ahadith). Ibn Sirin (d. 110), a Successor, said, "They would not ask about the isnad. But when the fitnah (trouble, turmoil, esp. civil war) happened, they said: Name to us your men. So the narrations of the Ahl al-Sunnah (Adherents to the Sunnah) would be accepted, while those of the Ahl al-Bid'ah (Adherents to Innovation) would not be accepted."