Every type of literature develops certain features particular to its nature and content and the character of the people who cultivate it, and the distinctive social, political or historical conditions in which it originates. and flourishes. Hadith literature is no exception to this rule. The isnad system was used to transmit pre-Islamic poetry.
However, it was in the hadith literature that it reached its peak at which it was considered an integral part of the religion itself. 'Abdullah ibn al-Mubarak (d. 181 A.H.), one of the illustrious teachers of Imam al- Bukhari, said:
"The isnad is from the religion. Were it not for the isnad anyone could say anything he wished." (Muslim)
By the end of the first century the science of isnad was fully developed. Ibn Sireen (d. 110), a Successor, said:
"(In the beginning) they would not ask about the isnad. But when the fitnah (turmoil/civil war) happened, they demanded, 'Name your men to us.' The narrations of the Ahlus-Sunnah (Adherents to the Sunnah) would be accepted, while those of Ahlu Bid'ah (Adherents to Innovations) would be rejected." (Muslim) That is, prior to the turmoil the isnad was used only occasionally, but after the turmoil they became cautious.
Other people began to copy the isnaad system from the Muslims. Professor Horovitz admitted that:
"In the Talmudic literature there is no idea of chronological method, and the oldest extant work attempting such an arrangement was composed after 885 A.D. - more than a century later than the earliest Islamic work on isnad-critique."
From this fact, he goes on to say:
"And from the fact that the important Jewish works (of this period) had been composed in the Islamic dominions, it may be inferred that this historical interest was due to the Islamic influence."
The practice of specifying the isnad, not only of the hadiths but also the books in which they were collected, was of immense value in preserving the integrity of books in an age in which printing was unknown, and the creation of spurious and distorted works was a relatively easy task.
The scholarly practice of certification seems to be unique in the world's literary history, just as hadith literature is unique in employing a thorough and systematic method of source identification. Greek, Latin, Hebrew and Syriac manuscripts rarely, if ever, supply us with such a wealth of information about a book's origin and use.
The isnad system, while originating in connection with the hadith literature, was in due course extended by Arab authors to many other genres, including geography, history, and prose fiction.