In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Sometimes a narration may come from someone from whom it is obligatory to accept, or from whom it is obligatory to reject, or from whom it is necessary to withhold from (until further clarification). And how foolish it would be, that that which is from the truth which the Seal of the Prophets conveyed from His Lord, the Mighty and Majestic and that which is from the falsehood that Allaah and His Messenger are free of, should be known by other than knowing the conditions of the narrators.
It is likewise for the various historical occurrences. In fact the need for knowing the condition of those who narrate them is even more severe, due to the prevalent neglect in the narration of such events and occurrences.
(And this is) in addition to the fact that being acquainted with condition of men is itself one of most important branches of history. So when it is necessary to be acquainted with the condition and state of the narrators, it is also necessary to make this known, so that everyone who knows the condition of a narrator should inform (others) about his condition, so that the people may know of it.
And this Ummah has (indeed) acted upon this obligation in the desired manner.
[Source: Ahimmiyatu Ilm ur-Rijal (translated Abu Iyaad Amjad Rafiq: for further details see: www.salafipublications.com article id: SLF020003]