Serenus, quoted by Stobaeus, Florilegium
, ed. A. Meineke, Vol. IV (Leipzig: Teubner, 1857), p. 205, tr. by Sir Thomas Heath, A History of Greek Mathematics
, Vol. I (Oxford: Clarendon Press, 1921; rpt. New York: Dover Publications, Inc., 1981), p. 357:
Some one who had begun to read geometry with Euclid, when he had learnt the first theorem, asked Euclid, "What shall I get by learning these things?" Euclid called his slave and said, "Give him threepence, since he must make gain out of what he learns."
Παρ' Εὐκλείδη τις ὰρξάμενος γεωμετρεῖν ὡς τὸ πρῶτον θεώρημα ἔμαθεν, ἤρετο τὸν Εὐκλείδην, "τί δέ μοι πλέον ἔσται ταῦτα μανθάνοντι;" καὶ ὁ Εὐκλείδης τὸν παῖδα καλέσας, "δός," ἔφη, "αὐτῷ τριώβολον, ἐπειδὴ δεῖ αὐτῷ ἐξ ὧν μανθάνει κερδαίνειν."