Hadith is the report of the words and deeds of Prophet Muhammad s.a.w. (peace be upon him) considered an authoritative source of revelation, second only to the Quran. Hadith were collected, transmitted, and taught orally for two centuries after Prophet Muhammad's (s.a.w.) departure and then began to be collected in written form and codified. They serve as a source of biographical material for Muhammad (saw), contextualization of Qur'anic revelations, and Islamic law.
A list of authoritative transmitters is usually included in collections. Compilers were careful to record hadith exactly as received from recognized transmission specialists. The six most authoritative collections are those of Bukhari, Muslim, Tirmidhi, Abu Dawud, Nasai and Ibn Majah. The collections of Malik ibn Anas ( Muwatta) and Ahmad ibn Hanbal ( Musnad Ahmad ) are also important.
The science of hadith criticism was developed to determine authenticity and preserve the corpus from alteration or fabrication. Chains of authority and transmission were verified as far back as possible, often to Prophet Muhammad (s.a.w.) himself. Chains of transmission were assessed by the number and credibility of the transmitters and the continuity of the chains (isnad). The nature of the text was also examined. Reports that were illogical, exaggerated, fantastic, or repulsive or that contradicted the Quran were considered suspect.
The complete hadith books can be downloaded for free from the links below.