One often reads “It is reported that the Prophet (peace and blessings be upon him) said…” or similar words. These reports are known as Hadiths (meaning “statements”, but often called Traditions in English). They have been handed down to us from the Companions, the Muslims who lived at the time of the Prophet.
Actually, a Hadith might be a report of what the Prophet (peace and blessings be upon him) said, did, or what he approved or disapproved of. The collection of the deeds, practices, and words of the Prophet—known from the Hadiths—is known as the Sunnah, and it forms the second source of Islamic Law after the Qur’an.
Muslims accept the Sunnah as equivalent to the Qur’an, the Book of Allah (God), and its ordinances are of equal weight to the ordinances of the Qur’an. This is because both the Qur’an and Sunnah are from Allah.
Sometimes the Qur’an gives a general principle, and the details are known only from the Sunnah. An example of this is the five daily ritual Prayers (Salah). The Qur’an instructs us to remember Allah at various times of the day, but the details of exactly when and how to perform Salah are known from the Sunnah, the words and actions of the Prophet himself (peace and blessings be upon him).
Some Hadiths were written down during the life of the Prophet, while others were passed on orally and collected later. Scholars study the Prophetic Hadiths and their chains of oral transmission to determine which are valid or authoritative. No Hadith is held true or cited as a proof unless it can be authentically attributed to the Prophet (peace and blessings be upon him).
There are six well known collections of Hadiths, the most authoritative of which are those by Al-Bukhari and Muslim. After a Hadith is quoted, it is common to cite the collection(s) from which it came. If the same Hadith appears in all of them, the words “agreed upon” or “the group” will usually follow it.
There are approximately 10,000 Hadiths. They are widely available in libraries and bookshops throughout the Muslim world. Some of the collections have been translated into English and other languages. But like the Qur’an, they can only be fully understood in Arabic, and only the Arabic text can be used in making religious rulings.