Hoyt Hopewell Hudson, The Epigram in the English Renaissance (Princeton UP, 1947), p. 115, identifies the original as itself a translation, from the seventh Epistola of Libanius, Lebanii Opera, ed. R. Foerster (Leipzig, 1921), vol. x, p. 3: Σὺ μὲν ἡμᾶς εἶπας κακῶς, ἡμεις δε καλῶς. ἀλλ᾽ οὔτε σοί τις οὔτ᾽ ἐμοὶ πείσεται.https://en.wikiquote.org/wiki/George_Buchanan