Hazrat Yahya bin Mu’aadh رحمه الله said; “Your brother is he who alerts you to your faults, and your true friend is he who warns you of your sins.
Hazrat Dawood Tai رحمه الله said; “Appoint an affectionate friend to constantly remind you of your faults. If he does not inform you of your faults, he cannot be your friend, for then your faults will remain with you and become known to others.”
(SOURCE: HAVEN OF SERENITY)