“As for me, all I know is that I know nothing.”

See All I know is that I know nothing on Wikipedia for a detailed account of the origins of this attribution.

μοι νυνὶ γέγονεν ἐκ τοῦ διαλόγου μηδὲν εἰδέναι· ὁπότε γὰρ τὸ δίκαιον μὴ οἶδα ὅ ἐστιν, σχολῇ εἴσομαι εἴτε ἀρετή τις οὖσα τυγχάνει εἴτε καὶ οὔ, καὶ πότερον ὁ ἔχων αὐτὸ οὐκ εὐδαίμων ἐστὶν ἢ εὐδαίμων.

Hence the result of the discussion, as far as I'm concerned, is that I know nothing, for when I don't know what justice is, I'll hardly know whether it is a kind of virtue or not, or whether a person who has it is happy or unhappy.

Republic, 354b-c (conclusion of book I), as translated by M.A. Grube in Republic (Grube Edition) (1992) revised by C.D.C. Reeve, p. 31

Confer Apology 21d (see above), Theaetetus 161b (see above) and Meno 80d1-3: "So now I do not know what virtue is; perhaps you knew before you contacted me, but now you are certainly like one who does not know."

Confer Cicero, Academica, Book I, section 1 http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A2007.01.0032%3Abook%3D1: "ipse se nihil scire id unum sciat ("He himself thinks he knows one thing, that he knows nothing"). Often quoted as "scio me nihil scire" or "scio me nescire." A variant is found in von Kues, De visione Dei, XIII, 146 (Werke, Walter de Gruyter, 1967, p. 312): "...et hoc scio solum, quia scio me nescire... [I know alone, that (or because) I know, that I do not know]." In the modern era, the Latin quote was back-translated to Greek as "ἓν οἶδα ὅτι οὐδὲν οἶδα", hèn oîda hóti oudèn oîda).

Confer Diogenes Laertius, II.32 (see above)
Misattributed