Plato, Meno 89a

μέλλει ἀγαθὰ εἶναι· καὶ τούτῳ τῷ λόγῳ φρόνησις ἂν εἴη τὸ ὠφέλιμον· φαμὲν δὲ τὴν ἀρετὴν ὠφέλιμον εἶναι; πάνυ γε. φρόνησιν ἄρα φαμὲν ἀρετὴν εἶναι, ἤτοι σύμπασαν ἢ μέρος τι; δοκεῖ μοι καλῶς λέγεσθαι, ὦ Σώκρατες, τὰ λεγόμενα. οὐκοῦν εἰ ταῦτα οὕτως ἔχει, οὐκ ἂν εἶεν φύσει οἱ ἀγαθοί. οὔ μοι δοκεῖ.

if they are to be good; and so by this account the profitable will be wisdom, and virtue, we say, is profitable? Certainly. Hence we conclude that virtue is either wholly or partly wisdom? It seems to me that your statement, Socrates, is excellent. Then if this is so, good men cannot be good by nature. I think not.