The scholars and narrators of hadith took great care in preserving the words of the Prophet ﷺ because of clear instructions from the Qur'an, direct commands and warnings from the Prophet himself, and combatting the trials that arose after his death.
يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوٓا۟ إِن جَآءَكُمْ فَاسِقٌۢ بِنَبَإٍۢ فَتَبَيَّنُوٓا۟ أَن تُصِيبُوا۟ قَوْمًۢا بِجَهَـٰلَةٍۢ فَتُصْبِحُوا۟ عَلَىٰ مَا فَعَلْتُمْ نَـٰدِمِين
[O you who have believed, if there comes to you a disobedient one with information, investigate, lest you harm a people out of ignorance and become, over what you have done, regretful.] [49:6]
With each chain of naration, there are levels to the names of men mentioned, and there are those that are more virtous than others. It would be helpful to define some of these terms:
If the narrator being examined is a Companion (Sahabi), then his narration is accepted without questioning, as it is inconceivable that the Companions — those who wept in prayer, feared the afterlife, and strictly followed the teachings of Islam — would ever lie about the Messenger ﷺ. Especially after hearing the severe warnings against fabricating statements about him.
However, if the narrator was other than a Companion, scholars would apply stricter scrutiny. They compiled detailed biographies in what became known as the "books of men" (Kutub al-Rijāl), recording each narrator's honesty, religious commitment, strength of memory, and the evaluations given by other scholars.
In this way, through analysing each narrator in the chain of a hadith, seeing whether or not they were trustworthy and reliable, starting from the one that compiled the hadith book, leading up to the Prophet ﷺ, scholars are able to determine which narrations are infact deemed authentic or otherwise.
In the next section, we will explore this science — known as Mustalah al-Hadith — and how scholars decided which narrations are authentic, good, weak, or even fabricated.