'The Isnad is part of the religion' proclaimed a Sunni contemporary of Shafi'i. 'If not for the Isnad, whoever wanted could say whatever they wanted'. Another early Sunni wrote preserving unbroken chains of transmission for their scriptures and sacred knowledge was what set Muslims apart from the followers of other religions. Shah WaliAllah devoted a whole book to celebrating this. 'If the Isnad had not been a basic principle' he wrote of Islam, 'then the Shariah would not have survived'.
In order to determine if an Isnad was reliable, Shafi'i proposed a critical method that had been developed by a network of ulama in Mecca, Medina, Iraq and northeastern Iran, all of whom specialized in Hadiths. Among them was Shafi'i's own teacher, Malik. Although not himself an important contributor to this science of transmission criticism, Shafi'i gives a useful summary of its methods. 'If a trustworthy person transmits [a Hadith] from another trustworthy person until the chain ends with the Messenger of God, then it is established as being from the Messenger of God'. To be trustworthy, Shafi'i explains, a Muslim had to be well known as honest, transmit a Hadith they had heard exactly as they heard it and be clear about the source from whom they heard it.
More importantly the Hadiths that this person recounted to others had to be corroborated by what other respected Hadith scholars also transmitted. If this person narrated Hadiths that broke with what others narrated, he or she could not be trusted. A chain of these reliable transmitters, each reporting the Hadith from their teacher, et cetera, had to extend unbroken to the prophet himself. During Shafi'i's time, these chains usually stretched from three to five or six people back to Muhammad. A break in the chain, like a person not remembering who told them the Hadith or quoting a person they never met, made the Hadith unreliable.
A Hadith that met these requirements was considered 'sound' (sahih). If it was widely transmitted through many circles of Hadith scholars, then it was also deemed 'well known' (mashhur). A Hadith with some flaw in its chain of transmission was termed 'weak' (da'if). It was the strength of the Isnad chain that determined if a Hadith should be heeded, since it meant that the Hadith had been authenticated as the words of Prophet. 'Misquoting Muhammad', Jonathan A.C Brown, Kindle, Loc 1020.
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