Hadith in religious use is often translated as 'tradition', meaning a report of the deeds and sayings of Muhammad. The Hadith literature does not qualify as "primary source" material as it was compiled from reports of hearsay that were present in society around the time of their compilation, which was well after the death of the Prophet Muhammad. Bukhari's collection which is considered by many traditional religious scholars as the most reliable was compiled two centuries after the death of the Prophet. Hadiths claims to originate from important characters of the earliest years of Islam such as the companions of Muhammad or Shia Imams/Religious leaders. These hadith narrations have formed the basis of "Islamic law", despite the contradictions they contain with regards to the Quran, for example the punishment for Zina in the hadith (stoning to death) contradicts the Quran and yet it is still the prescribed punishment by the established schools of thought such as the Hanafis. The Hadith has also had a profound influence on molding the commentaries (tafsir) on the Quran. Much of early Islamic history that is widely available is also based on the hadith.

Each hadith is composed of two parts, a chain of authorities reporting the hadith (isnad), which would not stand the tests of "reliability" according to modern standards of historical analysis, and the text itself (matn). Hadiths are regarded by traditional Islamic schools of jurisprudence as important tools for understanding the Quran and in matters of jurisprudence. Hadith were evaluated and gathered into large collections during the 8th and 9th centuries. These works are referred to in matters of Islamic law and history to this day.

The largest denominations of Islam, Sunni, Shiʻa, and Ibadi, rely upon different sets of hadith collections.

Clerics and jurists of all denominations classify individual hadith as sahih (authentic), hasan (good) and da'if (weak). However, different traditions within each denomination, and different scholars within each tradition, may differ as to which hadith should be included in which category.