Regard as your most faithful friends, not those who praise everything you say or do, but those who criticize your mistakes.
πιστοὺς ἡγοῦ μὴ τοὺς ἅπαν ὅ τι ἂν λέγῃς ἢ ποιῇς ἐπαινοῦντας ἀλλὰ τοὺς τοῖς ἁμαρτανομένοις ἐπιτιμῶντας.
Original text (Ancient Greek)