A hadis is said to be Sahih if it is not contrary to the other ahadis that have already been accepted by the authorities on the subject as authentic or reliable; nor should it contradict the text of the Quran, the consensus of the community Ijma, or the basic principles of Islam. It should not be against the dictates of reason or common sense, or the laws of nature. It should not level accusation against the family of the Prophet PBUH or his companions, or praise a particular person or tribe, should not be opposed to recognized historical facts. It should not mention any incident which, if it had happened would have been known and reported by other people while that incident was not reported by anyone except the particular reporter. Ít should not lay down punishments for minor offences or promise high rewards for insignificant good deeds or if the narrator himself confessed that he had fabricated the report.
The Sanad must be full and unbroken and each narrator mentioned in it must be known for his knowledge, piety, memory and trustworthiness. The narrators must have met each other and the chain of information must go back to the Holy Prophet pbuh, The best Sanad consists of those narrators who met the persons relating a Hadis and the person to whom they related it. The chain of transmission should be unbroken. The narrators should be at an age at which they could understand the importance of what they had heard. The report of a Child or an insane person could not be accepted. The narrator must be a Muslim. The higher the qualification of a narrator, the greater the authority of a tradition reported by him.
2) Approved or Hasan Hadis
This hadis is similar to an authentic hadis except that its narrators or some of its narrators have been found defective in memory in comparison with the narrators of authentic ahadis.
3) Weak or Zae’ef Hadis
A Hadis is considered to be weak if anyone of its narrators was known to be a liar or known to have bad memory. If he had not met the person he was supposed to be narrating from or there was a weakness in the text or the Sanad then the hadis was said to be a weak or Zae’ef Hadis.
4) Fabricated Hadis
When a saying or action was attributed to the Prophet pbuh which he had not said or done, then the hadis was said to be fabricated.