Plato, Meno 85b

ποσάπουν γίγνεται; ὀκτώπουν. ἀπὸ ποίας γραμμῆς; ἀπὸ ταύτης. ἀπὸ τῆς ἐκ γωνίας εἰς γωνίαν τεινούσης τοῦ τετράποδος; ναί. καλοῦσιν δέ γε ταύτην διάμετρον οἱ σοφισταί· ὥστʼ εἰ ταύτῃ διάμετρος ὄνομα, ἀπὸ τῆς διαμέτρου ἄν, ὡς σὺ φῄς, ὦ παῖ Μένωνος, γίγνοιτʼ ἂν τὸ διπλάσιον χωρίον. πάνυ μὲν οὖν, ὦ Σώκρατες. τί σοι δοκεῖ, ὦ Μένων; ἔστιν ἥντινα δόξαν οὐχ αὑτοῦ οὗτος ἀπεκρίνατο;

And how many feet is this space? Eight feet. From what line do we get this figure? From this. From the line drawn corner-wise across the (our-foot figure? Yes. The professors call it the diagonal: so if the diagonal is its name, then according to you, Meno’s boy, the double space is the square of the diagonal. Yes, certainly it is, Socrates. What do you think, Meno? Was there any opinion that he did not give as an answer of his own thought?