Imām Mālik - Allah have mercy on him - said:

Knowledge is not to be taken from four types of people: a foolish person who openly acts foolish, even if he reports the most narrations; an adherent of bid'ah who calls to his desires; a person who lies, even if I don't accuse him of lying in ḥadīth; and a righteous pious worshipper who does not accurately retain what he narrates.

Al-Dhahabī , Siyar A'lām Al-Nubalā' in his biography of Imām Mālik.