Plato, Republic, book 5 195:

φύλακασ ἡμῖν καὶ τὰσ φυλακίδασ, ἀλλά πῃ τὸν λόγον αὐτὸν αὑτῷ ὁμολογεῖσθαι ὡσ δυνατά τε καὶ ὠφέλιμα λέγει;