Allah ( SWT) said: “Verily, We have sent down the Reminder, and surely, We will guard it (from corruption)” [Al-Qur’an, Surah al-Hijr, 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 (May Allah bless him and grant him peace), for it is the practical example of the implementation of the Qur’anic guidance, the Wisdom taught to the Prophet sallallaahu `alayhi wa sallam ( may Allaah exalt his mention ) 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 those after them (may Allah be pleased with them) to memorize, write down and pass on the statements of the Messenger sallallaahu `alayhi wa sallam ( may Allaah exalt his mention ) and the descriptions of his way, as well as to continue the blessings of practising 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 sallallaahu `alayhi wa sallam ( may Allaah exalt his mention ) from those corrupted by weak memories, from forgeries by unscrupulous liars, and from the statements of the enormous number of ‘Ulama’, the Companions and those who followed their way, who had taught in various centers of learning and helped to transmit the legacy of Muhammad sallallaahu `alayhi wa sallam ( may Allaah exalt his mention ) - 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 renewers of Islam also revived the practice of the blessed authentic Sunnah.
Unfortunately however, statements will continue to be attributed to the Prophet sallallaahu `alayhi wa sallam ( may Allaah exalt his mention ) although the person quoting them may have no idea what the people of knowledge of Hadith have ruled regarding those Ahadith, 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 Muhaddithun (scholars of Hadith, “traditionists"). A Hadith (pl. Ahadith) is composed of two parts the Matn (text) and the Isnad (chain of reporters). A text may seem to be logical and reasonable but it needs an authentic Isnad with reliable reporters to be acceptable. ‘Abdullah Ibn al-Mubarak (d. 181 AH), one of the illustrious teachers of Imam al-Bukhari, said, “The Isnad is part of the religion had it not been for the Isnad, whoever wished to would have said whatever he liked”.

Among the sciences of Ahadith is the study of the chain of reporters (the Isnad) and many Muslim scholars have specialized in this field. It includes identifying the name of each and every narrator (reporter), his character (his truthfulness, piety, public behavior), his ability and reputation as a memorizer 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 (Sharia), or merely as a point of interest (not to be attributed to the sayings, etc. of the Prophet).

The Ahadith collected by Imams Bukhari and Muslim are all considered to be Sahih (authentic) by the Hadith experts. Both of these books contain only Sahih Hadith. However, there are other famous collectors of Hadith: Abu Dawud (d.275), al-Tirmidhi (d. 279) and al-Nasai (d. 303).