Hadith (literally means "talk" or "discourse") or Athar (Arabic: أثر, ʾAṯar, literally means "tradition") in Islam refers to what the majority of Muslims 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 hadees are transmitted reports about what Muhammad said and did. As noted by Emad Hamdeh, each report is a piece of data about Muhammad; when collected, these data points paint a larger picture which is referred to as the Sunnah.

Hadith has been called "the backbone" of Islamic civilization, and within that religion the authority of hadith as a source for religious law and moral guidance 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 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 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; many further claim that most Hadiths are fabrications (pseudepigrapha) created in the 8th and 9th century CE, and which are falsely attributed to Muhammad.

Because some hadith include questionable and even contradictory statements, the authentication of hadith became a major field of study in Islam. In its classic form a hadith has 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, hadīth 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)

There are basically six sacred Islamic books of Hadees (Hadith) known as Kutub al-Sitta (saha satta). These books contain collections of Islamic Hadees. These Islamic books of Hadith were compiled by six Muslim scholars in the ninth century. The books have been translated into various languages including Persian, English, Bangla, Tamil and Hindi etc. The Hadees included in these books have been explained in separate books known as Sharah. The Sharah of each Hadees book is written in numerous languages. The Hadees are divided into two types: Hadees e Nabvi and Hadees e Qudsi. The following is a list of six prominent canonical Hadees books:

Sahih Bukhari
Sahih Muslim
Sunan Abu Dawood
Jami AL-Tirimdhi
Sunan al-Sughra (Sunan an-Nasa'i)
Sunan Ibn Majah

Sahih Bukhari is one of the most prominent Hadees collections of Muslims. It was collected by a Persian Muslim scholar Muhammad Ibn Ismail AL-Bukhari (810–870 AD) 194–256 H (about 200 years after Muhammad died) and compiled during his life. The meaning of the Arabic word Sahih is authentic or correct.