Sahih hadith

Conditions

Ibn Hajar's definitions indicate that there are five conditions to be met for a particular hadith to be considered ṣaḥīḥ:

1. Each narrator in the chain of narration must be trustworthy;
2. Each narrator must be reliable in his ability to preserve that narration, be it in his ability to memorize to the extent that he can recall it as he heard it, or, that he has written it as he heard it and has preserved that written document unchanged;
3. The isnād must be connected (muttasil) insofar as it is at least possible for each narrator in the chain to have received the hadith from a predecessor;
4. The hadith, including its isnād, is free of ʻillah (hidden detrimental flaw or flaws, e.g. the establishment that two narrators, although contemporaries, could not have shared the hadith, thereby breaking the isnād.)
5. The hadith is free of irregularity, meaning that it does not contradict another hadith already established (accepted).

A number of books were authored in which the author stipulated the inclusion of ṣaḥīḥ hadith alone.

Ḥasan hadith

Ḥasan (حَسَن meaning "good") is used to describe hadith whose authenticity is not as well-established as that of ṣaḥīḥ hadith, but sufficient for use as supporting evidence.

Ibn Hajar defines a hadith that is ḥasan lithatihi – "ḥasan in and of itself" – with the same definition a ṣaḥīḥ hadith except that the competence of one of its narrators is less than complete; while a hadith that is ḥasan ligharihi ("ḥasan due to external factors") is determined to be ḥasan due to corroborating factors such as numerous chains of narration. He states that it is then comparable to a ṣaḥīḥ hadith in its religious authority. A ḥasan hadith may rise to the level of being ṣaḥīḥ if it is supported by numerous isnād (chains of narration); in this case that hadith would be ḥasan lithatihi ("ḥasan in and of itself") but, once coupled with other supporting chains, becomes ṣaḥīḥ ligharihi ("ṣaḥīḥ due to external factors").