Imaam Muslim (rahimahullaah) said:

know-may may Allaah (The Mot High) grant you success-that it is obligatory upon everyone (i.e. the scholars who criticise or commend narrators of hadeeth) to know the distinction between the authentic chains of transmission and the faulty ones, and the (trustworthiness, precision etc) of the narrators of (the authentic chains) as opposed to those who have been charged (with weakness, forgetfulness, fabrication etc in their narrations), so that nothing is transmitted from these narrations except that which whose narrator is sound and blameless in his narrations, and to avoid that which is from those who have been charged with (weakness, forgetfulness, fabrication etc) and the obstinate ones amongst Ahlul Bidah. [Muqaddimah Saheeh Muslim: page 61]