A hadith must meet the five criteria in order to be accepted according to Islaamic law as a source of legal ordinance. Ibn as-Salaah defined the Hadith Saheeh as follows:
“A Hadith Saheeh is one which has a continuous isnaad(chain), made up of trustworthy narrators narrating from trustworthy narrators, which is found to be free from any irregularities or defects.”
1. Ittisaal as-Sanad (Continuity of the chain of transmitters)
The chain of narrators or transmitters, who are relating the Matn (text), has to be unbroken for the hadith to be considered. That is none of the transmitters must be missing from the chain and each narrator, Raawee, has to have met the transmitter directly preceding him as well as the one directly following him. Each Raawee has to be a known individual, otherwise he is classified as majhool (unknown) and the sanad is classified as broken.
2. ‘Adaalah (integrity)
The integrity of the narrators is the second key condition for a hadith to be considered valid. By integrity we mean that the narrator was a practicing Muslim and was not known to have done any of the major (forbidden things) if he was a known liar he is classified as kaththaab and the hadith that he has transmitted is classified as da‘eef. These are the conditions verified through the references of the biographical science of hadith known as Kutub ar-Rijaal.
3. Dabt (accuracy)
The accuracy of the text is determined by two factors either of which is sufficient by itself.
(a) Dabt as-Sadr (Soundness of memory) Each narrator must be known for his ability to memorize and repeat with a high degree of accuracy. If a narrator had a tendency to repeat hadith in a number of different ways such a hadith is classified as Mudtarib (confused) and any other hadith that he narrates will be classified as Da‘eef. When the narrator’s level of accuracy is mediocre but the other conditions for authenticity are fulfilled, the hadith is classified as hasan.
(b) Dabt al-Kitaabah (Written accuracy) Each narrator who does not fulfil precondition “a”, must be known for recording his hadith in books accurately and his narrations only be from his books, these two preconditions (a,b) are also verified by Kutub ar-Rijaal (books on Biographies of narrators).
4. Ghayr Shaathth (conformity)
It is critical that the hadith conform to similar hadiths narrated on the same topic whose chains are stronger. If the text of a hadith contradicts that of another well-known text whose chain of narration is stronger, or it is in conflict with a group of other narrators of a similar status, it is classified as shaathth (errant), which is one of the categories of hadith da‘eef(weak).
5. Laa ‘Illah (absence of hidden defect)
The hidden defect is one that causes the hadith to appear to be sound and only become evident after deep investigation. For a hadith to be considered sound (saheeh) it has to be free of hidden defects. A hadith with hidden defects is called ma‘lool or mu‘allal. Ibn al-Madeenee (d. 324AH) said that a defect can only be revealed if all the isnaads of the hadith are collated. In his book, al-Ilal, he listed 34 Successors and the names of the Companions they heard hadiths directly from. For example, he said that al-Hasan al-Basree (d. 110AH) did not meet ‘Alee (d. 40AH), although there is a slight possibility that he may have seen him during his childhood in Madeenah. Such information is very important as it disproves the many Sufi traditions in which they claim that al-Hasan heard from ‘Alee. Only a few hadith scholars compiled books on this topic, among them, Ibn Abee Haatim ar-Raazee (d. 327), al-Khallaal (d. 311) and ad-Daaraqutnee (d. 385)
Ruling of Hadith Saheeh
A hadith that fulfils all the five conditions of sihhah(authenticity) is referred to as a hadith Saheeh. Such a hadith can be used to establish points of Islamic law and, if it isn’t abrogated, it must be accepted and applied. The ruling of a hadith saheeh can only be superceded by that of another hadith saheeh stronger than it.
Taken from the book Usool Al-Hadith by Abu Ameenah Bilal Phillips