How do you know which are authentic and which aren't?
The science of hadith is a very big story. There are specialists in this science who laid down certain criteria and conditions to accept a certain hadith as authentic.
The hadith consists of two basic elements;
sanad and
matn.
The sanad means the chain of narrators, which starts with the person who heard the hadith from the Prophet, then the person who heard from him and so on.
The matn is the actual wording that the Prophet said.
A Hadith must meet the following five criteria in order to be accepted in Islamic law as a source of legal ordinance:
1. Continuity of the chain of transmitters (ittisal assanad):
This chain of transmitters has to be unbroken in order for the Hadith to be acceptable. That is, none of the transmitters must be missing from the chain of narrators. Furthermore, each transmitter must also have heard the Hadith in question directly from the transmitter before him. Knowledge of this is verified with the help of the biographical sciences of the science of Hadith.
2. The integrity ('adalah) of the transmitters:
The integrity of the transmitters is established in terms of their out- ward observance of Islam. In other words, it is ascertained that they practice what is required of them by Islam and they are not known to engage in the doing of things which are forbidden. Again this precondition is verified through the biographical sciences of Hadith.
3. Soundness of memory of the transmitters:
It must be verified through the biographical sciences of Hadith that each transmitter has a sound memory or that his books were accurate and that he only transmitted directly from his books.
4. Conformity of the Hadith:
It is important that the Hadith conform with similar Hadiths on the same topics which are stronger than it. This conformity should be both in the chain of transmitters and the text. Non-conformity in the chain of transmit- ters for example, might be if one of the transmitters in the chain is differ- ent than in a stronger version of the same Hadith. Non-conformity in text would imply divergence in the meaning of this Hadith with one which is stronger.
5. The absence of defects ('illah) in the Hadith:
A defect ('illah) in Hadith is defined as a hidden defect in the Hadith which takes away from its authenticity. A Hadith which has such a defect is one which appears to be free from defect at first while after investiga- tion it is discovered that it has a certain defect which would not be apparent without investigation. The defect can be in the chain of transmitters or in text or both.
Based on this the hadith went into different classifications and levels.
The Classification Of Hadith:
1. According to the reference to a particular authority
2. According to the links in the isnad -- interrupted or uninterrupted
3. According to the number of reporters in each stage of the isnad
4. According to the manner in which the hadith is reported
5. According to the nature of the text and isnad
6. According to a hidden defect found in the isnad or text of a hadith
7. According to the reliability and memory of the reporters
With regards to its authenticity, it can be categorized into 5 types, 1)
Sahih (authentic), 2)
Hasan (sound), 3)
Dha'eef (weak), 4)
Dha'eef Jiddan (very weak), and 5)
Mawdhoo (fabricated).
There is an important science called "Ilm al Rijal" (knowledge of men). It's concerned with the study of the narrators of the hadith; when they were born, died, where they traveled, their memory, their beliefs, how religious they were, their scholars...etc. It aims to classify the hadith and distinguish what's authentic from what's not.
Sources:
History of Sunnah: Brief overview of the history of the Sunnah, how it was transmitted and how it was preserved. Exposing lies about the Sunnah.
Hadith: Rules For Acceptance And Transmission
Q & A: Hadith Categories Based on Authenticity
Science Of Hadith