The system: Every hadith (reported saying or action of Muhammad) must be accompanied by an isnad — an unbroken chain of named transmitters going back to the Prophet or a Companion. Format: ‘A told me that B told him that C heard from D who heard the Prophet say…’ The science of rijal (‘men’) evaluates the reliability of every transmitter in the chain based on their character, memory, dates, and whether they could have actually met the people they claimed to transmit from.

Theological significance: The isnad system is Islam’s answer to the question ‘how do we know what Muhammad actually said?’ It creates a documentary epistemology: truth is established through authenticated chains of human testimony, not through mystical experience, allegorical interpretation, or philosophical reasoning alone. This has profound implications: (1) It empowers hadith scholars as the arbiters of authentic tradition, creating a scholarly class with authority parallel to (and sometimes over) rulers; (2) It creates the possibility of authenticating or rejecting specific traditions — the hadith collections of Bukhari and Muslim were considered most reliable (sahih), but thousands of hadiths were classified as weak (daif) or fabricated (mawdu’). (3) The Quran alone (Quraniyya) movement rejects hadiths entirely, arguing the isnad system cannot be verified and that the Quran is sufficient — a position mainstream Sunni Islam treats as heretical deviation from the Sunnah.