Hadith (Arabic: الحديث) which is also called "riwaya" ( lit.: narration) is a word in Islamic terminology and religious sciences which refers to sayings quoted from the Prophet (s) and Imams (a) and their conduct.

Beside the Qur'an, hadith also has had a central role in Muslims' understanding of religion during history of Islam.

Due to its importance, different sciences have been established to study hadiths from different aspects of reference, content, etc. under the general title of hadith sciences. Issues about the sources of the hadiths are studied in riwayat al-hadith and their content is studied in dirayat al-hadith. rijal and mustalah al-hadith are two branches of riwayat al-hadith. Different branches have been established under the mentioned disciplines to more specifically analyze and check authenticity of hadiths.

Among the many hadith books, ten books are more important. Four of them are written by Shi'a and other six ones are written by Sunnis. The four most important Shi'a books are al-Kulayni's al-Kafi, al-Shaykh al-Tusi's Tahdhib al-ahkam and al-Istibsar fi ma ikhtalaf min al-akhbar and al-Shaykh al-Saduq's Man la yahduruh al-faqih. These books are called al-Usul al-'Arba'a or al-Kutub al-'Arba'a (The Four Books).

To Sunnis, most authentic books are Sahih al-Bukhari, Sahih Muslim (which are called Sahihayn), Sunan Abi Davud, Sunan al-Tirmidhi, Sunan al-Nasa'i and Sunan Ibn Maja. These books are called al-Sihhah al-Sitta(6).