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,
الْإِسْنَادُ مِنْ الدِّينِ وَلَوْلَا الْإِسْنَادُ لَقَالَ مَنْ شَاءَ مَا شَاءَ
"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."
FOOTNOTE: 1. Ar. Sunnah: Way, Path, Tradition, Example. See An Introduction to the Sunnah by Suhaib Hasan (Understanding Islam Series no. 5, published by Al-Quran Society), for Qur'anic proofs of revelation besides the Qur'an, the importance of the Sunnah, and a brief history of the collections of Hadith. See also Imam al- Shafi'i's al-Risalah for the authoritative position of the Sunnah (Eng. trans., pp. 109- 116).
2. Related by Imam Muslim in the Introduction to his Sahih - see Sahih Muslim (ed. M.F. 'Abdul Baqi, 5 vols., Cairo, 1374/1955), 1:15 & Sahih Muslim bi Sharh an-Nawawi (18 vols. in 6, Cairo, 1349), 1:87. The existing English translation of Sahih Muslim, by Abdul Hamid Siddiqi, does not contain this extremely valuable Introduction.
3. Ibn Abi Hatim al-Razi, Al-Jarh wa l-Ta'dil (8 vols., Hyderabad, 1360-1373), 1:20.
4. Sahih Muslim, 1:15. See Suhaib Hasan, Criticism of Hadith among Muslims with reference to Sunan Ibn Maja (Ta Ha publishers / Al-Quran Society, London, 1407/1986), pp. 15-17 for discussion of this statement of Ibn Sirin.