Book 7 Proposition 35
Ἐὰν δύο ἀριθμοὶ ἀριθμόν τινα μετρῶσιν, καὶ ὁ ἐλάχιστος ὑπ' αὐτῶν μετρούμενος τὸν αὐτὸν μετρήσει. Δύο γὰρ ἀριθμοὶ οἱ Α, Β ἀριθμόν τινα τὸν ΓΔ μετρείτωσαν, ἐλάχιστον δὲ τὸν Ε: λέγω, ὅτι καὶ ὁ Ε τὸν ΓΔ μετρεῖ. Εἰ γὰρ οὐ μετρεῖ ὁ Ε τὸν ΓΔ, ὁ Ε τὸν ΔΖ μετρῶν λειπέτω ἑαυτοῦ ἐλάσσονα τὸν ΓΖ. καὶ ἐπεὶ οἱ Α, Β τὸν Ε μετροῦσιν, ὁ δὲ Ε τὸν ΔΖ μετρεῖ, καὶ οἱ Α, Β ἄρα τὸν ΔΖ μετρήσουσιν. μετροῦσι δὲ καὶ ὅλον τὸν ΓΔ: καὶ λοιπὸν ἄρα τὸν ΓΖ μετρήσουσιν ἐλάσσονα ὄντα τοῦ Ε: ὅπερ ἐστὶν ἀδύνατον. οὐκ ἄρα οὐ μετρεῖ ὁ Ε τὸν ΓΔ: μετρεῖ ἄρα: ὅπερ ἔδει δεῖξαι.
If two numbers measure any number, the least number measured by them will also measure the same. For let the two numbers A, B measure any number CD, and let E be the least that they measure; I say that E also measures CD. For, if E does not measure CD, let E, measuring DF, leave CF less than itself. Now, since A, B measure E, and E measures DF, therefore A, B will also measure DF. But they also measure the whole CD; therefore they will also measure the remainder CF which is less than E: which is impossible.
See Textual Note on El. 7.21.