The isnad (Arabic) are the citations or "backings" that establish the legitimacy of the hadith, which are the sayings of Muhammad, Prophet of Islam.
After Muhammad's death in 631, Islam began to divide into factions based on different interpretations of his views. It became important to trace narrators to the source, and to investigate the reputation and reliability of that source for transmitting evidence - this developed into Ilm ar-rijal or the "science of biography".
An isnad is in the form "A said that B said that C wrote (in a lost work) that D read in (a work known to exist but also lost) E that Muhammad had said...", where A, B, C, D, E were known figures with known histories. The focus of these scholars was to determine if in fact these individuals could have met, under what circumstances or social pressures, if translation were involved, if lost records are involved were they actually likely to have been lost, etc., and therefore to come to their own conclusions about the validity.
Scholars emerged who devoted their lives to checking each link in the chain. Questions they asked about the isnah included:
- are these individuals reliable reporters?
- could these individuals have met, given where they were in time and space?
- is there any record of their meeting or collaborating or having any common interests?
- are the individuals of sound morals and not motivated by politics or factional concerns of sects?
- is the reported tradition logically consistent? is it actually rational?
- does it linguistically reflect the words of the Prophet, in his vocabulary?
- does the reported tradition agree with the Qur'an?
- is it the kind of matter or thing which we can reasonably believe Muhammad to have said?
- Sahih Bukhari, 7275 authentic out of six million reportedly reviewed by Imam Bukhari (d. 870)
- Sahih Muslim, 9200 authentic out of three million reported reviewed by Imam Muslim (d. 875)
- Sahih Tirmidhi
- Sahih Ibn Majah
- Sahih Abu Dawud
- Sahih An-Nisai
Isnad was influential in the development of disciplined scientific citation as early Muslim philosophy developed and applied Muslim disciplines like isnad and ijtihad and ijma to the natural world. Some claim this resulted in the breakthroughs in early Muslim medicine and the Mutazilite school of scientists. However, the capacity to cite prior authority so reliably was probably also influential in the rise of the Asharite school, which led to the classical fiqh and taqlid "blind imitation" of prior jurists, and ultimately limited Muslim sciences.