It goes without saying that Prophet Muhammad (peace be upon him) has left us with two powerful sources for the improvement of the individual, the society and for the nations, those are: the Holy Quran and ahadith. The Quran was revealed to the Last Messenger (peace be upon him) through the angel Gabriel. The ahadith show us the diversities a single person can have, and the ways these diversities may be used to produce an optimal performance, where the performance is exemplary for future generations. This was one of the main tasks of the Last Messenger (peace be upon him), to guide humanity on the right path.
Muhammad (peace be upon him) was a son, a husband, a father, an advisor, a warrior, a leader, a judge and most important of all, he was the Seal of the Prophets - the last messenger of Allah. A man with such a character and such a vast field of responsibility is indeed deserving of biographical study, and through the ages, biographical researches have been carried out.
The collection of ahadith is the result of these researchers. The study of ahadith is a science itself. To mention some branches within the study of ahadith, there is the science of Asma'ul Rijal (the science of study of the life-history of 500,000 narrators), the science of Mustalih al-Hadith (the science of classifying a hadith as authentic or weak), the science of Gharib al-hadith (the study of the linguistic origins of the difficult words used in ahadith).
I have mentioned only three different sciences though this list can extend to over 100. This again indicates the immense importance of ahadith and the importance of understanding them.
The main source of ahadith can be found in six sahihs (NOTE C) These six sahihs are:
Sahih al-Bukhari
(d. 256 A.H. = 870 C.E.)
Sahih Muslim
(d. 261 A.H. = 875 C.E.)
Sunan Ibn Majah
(d. 273 A.H. = 887 C.E.)
Sunan Abu Dawud
(d. 275 A.H. = 888 C.E.)
Jami al-Tirmidhi
(d. 279 A.H. = 892 C.E.)
Sunan al-Nisai
(d. 303 A.H. = 915 C.E.)
A.H. = After Hegira C.E. = Christian Era
