Ḥadīth ( or ; Arabic: حديث, ḥadīṯ, Arabic pronunciation: [ħadiːθ], pl. aḥādīth, أحاديث, ʾaḥādīṯ, Arabic pronunciation: [ʔaħadiːθ], literally "talk" or "discourse") or Athar (Arabic: أثر, ʾAṯar, literally "remnant"/"effect") refers to what most Muslims and the mainstream schools of Islamic thought, believe to be a record of the words, actions, and the silent approval of the Islamic prophet Muhammad as transmitted through chains of narrators. In other words, the ḥadīth are transmitted reports attributed to what Muhammad said and did.

Hadith have been called by some as "the backbone" of Islamic civilization, and for many the authority of hadith as a source for religious and moral guidance known as Sunnah, which ranks second only to that of the Quran (which Muslims hold to be the word of God revealed to Muhammad). Most Muslims believe that scriptural authority for hadith comes from the Quran, which enjoins Muslims to emulate Muhammad and obey his judgements (in verses such as 24:54, 33:21).

While the number of verses pertaining to law in the Quran is relatively few, hadith are considered by many to give direction on everything from details of religious obligations (such as Ghusl or Wudu, ablutions for salat prayer), to the correct forms of salutations and the importance of benevolence to slaves. Thus for many, the "great bulk" of the rules of Sharia (Islamic law) are derived from hadith, rather than the Quran.

Ḥadīth is the Arabic word for things like speech, report, account, narrative.^: 471 Unlike the Quran, not all Muslims believe that hadith accounts (or at least not all hadith accounts) are divine revelation. Different collections of hadīth would come to differentiate the different branches of the Islamic faith. Some Muslims believe that Islamic guidance should be based on the Quran only, thus rejecting the authority of hadith; some further claim that most hadiths are fabrications (pseudepigrapha) created in the 8th and 9th centuries AD, and which are falsely attributed to Muhammad.

Because some hadith contain questionable and even contradictory statements, the authentication of hadith became a major field of study in Islam. In its classic form a hadith consists of two parts—the chain of narrators who have transmitted the report (the isnad), and the main text of the report (the matn). Individual hadith are classified by Muslim clerics and jurists into categories such as sahih ("authentic"), hasan ("good"), or da'if ("weak"). However, different groups and different scholars may classify a hadith differently.

Among scholars of Sunni Islam the term hadith may include not only the words, advice, practices, etc. of Muhammad, but also those of his companions. In Shia Islam, hadith are the embodiment of the sunnah, the words and actions of Muhammad and his family, the Ahl al-Bayt (The Twelve Imams and Muhammad's daughter, Fatimah).