Plato, Republic, book 2 11:

τρίτον δὲ ὁρᾷσ τι, ἔφη, εἶδοσ ἀγαθοῦ, ἐν ᾧ τὸ γυμνάζεσθαι καὶ τὸ κάμνοντα ἰατρεύεσθαι καὶ ἰάτρευσίσ τε καὶ ὁ ἄλλοσ χρηματισμόσ;