Definition of "hadith:"

Hadiths are regarded as a narration on the Sunnah (lived example) of Muhammad.  It includes reported sayings, actions, and traditions of Mohammad and his companions.
M.M. Azami formally defines \"hadith\" as follows:

\"According to Muhaddithiin [scholars of hadith] it stands for
\'what was transmitted on the authority of the Prophet, his deeds,
sayings, tacit approval, or description of his sifaat (features)
meaning his physical appearance. However, physical appearance of
the Prophet is not included in the definition used by the
jurists.\' \"
\"Thus hadith literature means the literature which
consists of the narrations of the life of the Prophet and the
things approved by him. However, the term was used sometimes in
much broader sense to cover the narrations about the Companions
[of the Prophet] and Successors [to the Companions] as well.\"

The Qur\'an says: \"And
whatever the Messenger gives you, take it, and whatever he
forbids you, leave it. And fear Allah: truly Allah is severe in
punishment.\"  The Messenger
refers here to the Prophet Muhammad. These writings are not
regarded as having the same status as the Holy Qur\'an, which is
considered to be God\'s word.
The accuracy of Muhammad\'s sayings was
confirmed by his contemporaries -- generally his companions; i.e.
his immediate followers. Some Muslims regard all of the the Hadiths
as being valid. Some historians question the accuracy of some
passages. For example, Historian Bernard Lewis commented on a saying
attributed to the prophet that some scholars believe is invalid:

\"If anyone insults me, then any Muslim who hears this must kill him immediately.\"

Some
terrorists partly believe that, based on this Hadith, all Muslims have a
duty to kill Americans when they have the opportunity.
The great
Islamic scholar Yahya bin Sharaf Ul-Deen An-Nawawi compiled a collection
of 43 of the most important sayings of Prophet Muhammad (PBUH).
It is is now known as \"An-Nawawi\'s Forty Hadiths\"