Reasons to Reject Narrators of Ahadith
As you know, we cannot just accept a hadith from anyone–this is our deen! If anyone could just say what they wanted, we would have nothing left of the true, pure message of Allah.
The scholars have analyzed the biographies of every narrator of hadith. They compile information on when he was born, and where, his kunya, his tribe, who he met, what his character was, what other people narrated about him … piles and piles of information.