Plato, Meno 96b

εἶναι περὶ αὐτὸ τοῦτο τὸ πρᾶγμα οὗ φασι διδάσκαλοι εἶναι, οἱ δὲ ὁμολογούμενοι αὐτοὶ καλοὶ κἀγαθοὶ τοτὲ μέν φασιν αὐτὸ διδακτὸν εἶναι, τοτὲ δὲ οὔ; τοὺς οὖν οὕτω τεταραγμένους περὶ ὁτουοῦν φαίης ἂν σὺ κυρίως διδασκάλους εἶναι; μὰ Δίʼ οὐκ ἔγωγε. οὐκοῦν εἰ μήτε οἱ σοφισταὶ μήτε οἱ αὐτοὶ καλοὶ κἀγαθοὶ ὄντες διδάσκαλοί εἰσι τοῦ πράγματος, δῆλον ὅτι οὐκ ἂν ἄλλοι γε; οὔ μοι δοκεῖ.

and indeed as inferior in the very quality of which they claim to be teachers; while those who are themselves recognized as men of worth and honor say at one time that it is teachable, and at another that it is not? When people are so confused about this or that matter, can you say they are teachers in any proper sense of the word? No, indeed, I cannot. Well, if neither the sophists nor the men who are themselves good and honorable are teachers of the subject, clearly no others can be? I agree.