The early writers on the subject of the hadith evolved certain rules to classify the hadith. There are two main types of classification in use. One deals with the degree of the authenticity of the tradition, while the other takes as its base the way the tradition was transmitted.
As far as the first classification is concerned, the traditionists have divided the traditions into three classes, according to the degree of reliability based on the perfection or imperfection of the chain of their transmitters. Also, they saw whether the texts had any hidden defects. The acceptance or rejection of the particular hadith by the Companions, the Followers and their Sucessors was also taken into consideration.
These three classes are:
a) Genuine (Sahih) – this name is given to a faultless hadith, in which there is no weakness either in regard to the chain of transmission (isnad) or in regard to the text (matn), and in which there is no contradiction of any kind of any of the established beliefs of Islam.
b) Fair (Hasan) – this hadith is similar to Sahih hadith only some of its narrators might have to be found to have weaker or defective memory as compared to the narrator of Sahih hadith.
c) Weak (dhaif) or – this is a tradition, in respect of which some serious doubts can be raised. These doubts might be in respect to its content or the text, or because one or more of its transmitters are considered unreliable.
d) Forged (Maudu) – this is a totally forged hadith.
The writers on the Science of Hadith as well as the jurists, have also divided the traditions according to the number of their transmitters during the first three generations of the Muslims, into three types: mutawatir, mashhur and ahad.
1. Mutawatir – these are the traditions which have been transmitted throughout the first three generations of the Muslims by such a large number of transmitters that there is no doubt that the hadith is genuine.
2. Mashhur – these are the traditions, which, having been originally transmitted in the first generation by two, three or four transmitters, were later on transmitted on their authority, by a large number of transmitters in the next two generations.
3. Ahad – There are the traditions, which were transmitted during the first three generations of the Muslims by one to four transmitters only.