Hadith: The principle that the hadith possesses validity, as attested by the Quran, is not at all disputed among Shi'ites or in fact among all Muslims. But because of the failure of some of the early rulers of Islam in preserving and guarding the hadith, and the excesses of a group among the companions and followers of the Prophet in propagating hadith literature, the corpus of hadith came to face a certain number of difficulties.
On the one hand the caliphs of the time prevented the writing down and recording of the hadith and ordered any pages containing texts of hadith to be burned. Sometimes also any increase in activity in the transmission and study of hadith was forbidden. In this way a certain number of hadiths were forgotten or lost and a few were even transmitted with a different or distorted meaning. On the other hand another tendency also prevailed among another group of the companions of the Holy Prophet who had had the honor of seeing his presence and actually hearing his words. This group, which was respected by the caliphs and the Muslim community, began an intense effort to propagate the hadith. This was carried to such an extent that sometimes hadith overruled the Quran and the injunction of a Quranic verse was even considered abrogated by some people through a hadith. Often the transmitters of hadith would travel many miles and bear all the difficulties of traveling in order to hear a single saying.
A group of outsiders who had worn the dress of Islam and also some of the enemies within ranks of Islam began to change and distort some of the hadith and thus diminished the reliability and validity of the hadith that was then heard and known. For this very reason Islamic scholars began to think of a solution. They created the sciences concerned with the biography of learned men and chains of transmission of hadith in order to be able to discriminate between true and false hadith.