Of all the works of Hadith, Saheeh Al-Bukhari and Saheeh Muslim are regarded as the most authentic and authoritative books, after Al-Qur’an. Indeed the very word “Saheeh” means “authentic”.
Saheeh Al-Bukhari was compiled by Imam (leader) of Hadith, Mohammad Ibn Ismaa’eel al-Bukhari, born 194AH in Bukhara, central Asia. He travelled at an early age seeking knowledge to Hijaaz (Makkah and Madinah), Ancient Syria, Iraq, Egypt, etc. He devoted more than 16 years of his life to the actual compilation of this work. He learnt from more than 1,000 scholars.
It is said that Imam Al-Bukhari  collected over 300,000 Hadiths and he himself memorised 200,000 of which some were unreliable. He wrote many books especially on the bibliography of Hadith narrators and other books on various issues of Fiqh (Islamic jurisprudence). However, his book Saheeh Al-Bukhari, stands out for being the most authentic book of Hadith. It was also the first book to contain only authentic Hadiths, while previous books contained authentic and non-authentic Hadiths, as well as sayings of the companions  and others. Imam Al-Bukhari  died in his hometown, Bukhara in the territory of Khurasaan (West Turkistan), in the year 256AH. 
He grouped the traditions of the Prophet, sallallaahu ‘alaihi wa sallam, under various headings dealing with specific points of Islamic jurisprudence. In his time, the schools of law had been generally established and his objective was to catalogue the traditions he regarded as authentic in relation to their respective topics of jurisprudence.
Before he recorded each Hadith, he would perform ablution and offer a two-Rak’ah prayer and supplicate to Allah. Many religious scholars of Islam tried to find fault in the great remarkable collection, but without success. It is for this reason, they unanimously agreed that the most authentic book after the Book of Allah is Saheeh Al-Bukhari. 
Some facts about Saheeh Al-Bukhari:
1. It contains 7,275 Hadiths, which he chose from the large number of Hadiths that he had collected.
2. The number of complete unrepeated Hadiths is 2,230.
3. All the Hadiths mentioned are authentic.
4. The conditions for accepting a Hadith were very stringent. Such as:
a) The chain of narrators must be linked, i.e. every narrator must have met his predecessor, (the man of whom he heard the Hadith from, up to the Prophet, sallallaahu ‘alaihi wa sallam.)
b) For it to be enclosed in the Saheeh, the narrators must be of the highest caliber regarding their piety, manners, memory, integrity, etc. 
5. The book is not a mere book of narrations; it is essentially a course of study on Hadith, its derivatives, inductions and research.
6. Each one of its 97 chapters is headed by a relevant verse from the Qur’an that complements the meaning of the Hadiths quoted. 
7. Finally, much more could be said about this monumental work, however, it is enough to say that many people have reached fame and achieved the highest qualifications by studying the book, researching it and commenting on it.
Saheeh Muslim:
Saheeh Muslim is the second most authentic book of Hadith after Saheeh Al-Bukhari, compiled by Imam Muslim ibn al-Hajaaj al-Nisapuri. Born in 202H and died in 261H. He travelled widely to gather his collection of Hadith to Iraq Hijaaz, Ancient Syria, and Egypt. He learnt from many scholars, most of which were Al-Bukhari’s teachers. He also learnt from the Imam Al-Bukhari himself  and became his most loyal student. Like Imam Al-Bukhari, Allah have mercy upon him, he, Allah have mercy upon him, wrote many books on the sciences of Hadith. 
He, Allah have mercy upon him, sought not so much to complement the issues at stake in the fiqh, the lslamic jurisprudence, but rather to produce a collection of sound traditions, an authentic record, on which future studies of Hadith could be based. 
Some facts about Saheeh Muslim:
1. The book contains 4,000 non-repeated Hadiths and 12,000 repeated ones.
2. Many narrations are mentioned in Saheeh Al-Bukhari, but with different chain of narrators.
3. In every chapter more than one Hadith with the same meaning but with different chains and text are listed. The first Hadith in each chapter is the strongest, followed by weaker narrations in order to strengthen weaker narrations.
4. Excellent classification.
5. The book is forwarded by a detailed introduction about the basis of the sciences of Hadith.
Saheeh Al-Bukhari is preferred over Saheeh Muslim based on the authenticity of the Hadiths. Imam Al-Bukhari was stricter in selecting Hadiths (chains) than Imam Muslim . Besides considering all the conditions of a Saheeh Hadith, Imam Al-Bukhari  stipulated a further condition that a narrator should meet the person from whom he is narrating the Hadith. 
Imam Muslim  however, did not stipulate the evidence of meeting the narrator from whom he is narrating, but according to him, it is sufficient to accept the Hadith of a narrator if he lives in the same period and there was the possibility of meeting the narrator from whom he is reporting the Hadith. So, the condition of Imam Muslim was less strict than the condition of Imam Al-Bukhari .  
While Bukhari’s compilation is considered the more reliable of the two, Muslim’s arrangement of his material has been recognised as superior, and rightly so. While Al-Bukhari  made the traditions in his collection testify to his own schedule of various points of law, Muslim  left them to speak for themselves.

Article source: http://www.islamweb.net/emainpage/

The narrators of Hadith

The narrators of Hadith are the most honourable and respectable Muslims, as they are keepers and preservers of Sunnah. Most of them are the companions of the Prophet, sallallaahu ‘alaihi wa sallam, who witnessed the acts and learnt the sayings of the Prophet. Others are the successors of the companions. Who were blessed by Allah and they are highly respected by all Muslims. They are shinning stars of Islam. 
Their hearts were free from worldly desires. Reading their biographies will increase love and devotion towards them and develop more interest to be faithful and obedient in the heart of the Muslim who reads their biography.
Allah Almighty Says (what means): “Verily, We have sent down the Reminder, and surely, We will guard it (from corruption).” [Qur’an; 15:9] The above promise made by Allah is obviously fulfilled in the undisputed purity of the Qur’anic text throughout the fourteen centuries since its revelation. 
However, what is often forgotten by many Muslims is that the above divine promise also includes, by necessity, the Sunnah of the Prophet Muhammad, sallallaahu ‘alaihi wa sallam,  for it is the practical example of the implementation of the Qur’anic guidance, the Wisdom taught to the Prophet  along with the Scripture, and neither the Qur’an nor the Sunnah can be understood correctly without recourse to the other. 
Allah preserved the Sunnah by enabling the Companions and their followers to memorise, write down and pass on the statements of the Messenger of Allah and the descriptions of his way, as well as to continue the blessings of practicing the Sunnah. Later, as the purity of the knowledge of the Sunnah became threatened, Allah caused the Muslim nation to produce outstanding individuals of incredible memory-skills and analytical expertise, who journeyed tirelessly to collect hundreds of thousands of narrations and distinguish the true words of precious wisdom of their Messenger  from those corrupted by weak memories, from forgeries by unscrupulous liars, and from the statements of the enormous number of scholars, the Companions and those who followed their way, who had taught in various centers of learning and helped to transmit the legacy of Muhammad  - all of this achieved through precise attention to the words narrated and detailed familiarity with the biographies of the thousands of reporters of Hadith. Action being the best way to preserve teachings, the scholars of Islam also revived the practice of the blessed authentic Sunnah. 
Unfortunately, however, statements will continue to be attributed to the Prophet, sallallaahu ‘alaihi wa sallam, although the person quoting them may have no idea what the people of knowledge in the field of Hadith science have ruled regarding those Hadiths, thus ironically being in danger of contravening the Prophet’s widely-narrated stern warnings about attributing incorrect/unsound statements to him. 
The methodology of the expert scholars of Hadith in assessing narrations and sorting out the genuine from the mistaken, fabricated etc., forms the subject-matter of a wealth of material left to us by the scholars of Hadith (traditionists). 
A Hadith is composed of two parts: the Matn (text) and the Isnaad (chain of reporters). A text may seem to be logical and reasonable but it needs an authentic Isnaad with reliable reporters to be acceptable. ‘Abdullaah Ibn al-Mubaarak  (d. 181AH), one of the illustrious teachers of Imam Al-Bukhari  said: “The Isnaad is part of the religion, had it not been for the Isnaad, then people would have claimed whatever they wished.” 
Among the sciences of Hadith is the study of the chain of reporters (the Isnaad). Many Muslim scholars have specialised in this field. It includes identifying the name of each and every narrator (reporter), his character (his truthfulness, piety, public behaviour), his ability and reputation as a memoriser and the types of narrations he is known to report, whether authentic, weak, fabricated, etc. In addition, each narrator should be identified by a rating given by other narrators who knew him. So all of these and many other details must be considered to know the degree to which a Hadith may be used as a basis for Islamic belief or practice (Shari’ah), or merely as a point of interest (not to be attributed to the sayings, etc. of the Prophet). 
After the Book of Allah (The Qur’an), the books of Hadith collection that were collected by Imams Bukhari and Muslim  are considered by the Muslim scholars to be the most authentic books of Hadith. However, there are other famous scholars in the field who compiled books of Hadith such as; Abu Daawood (d.275), At-Tirmithi (d. 279), An-Nasaa’i (d. 303) and others.