This is from Xenophon's Hellenica (7.1.2). Ambassadors from the Spartans are in Athens hoping to make a treaty:
ἐὰν οὖν ᾗ ἑκατέροις μάλιστα συνοίσει,
if therefore the means by which to each it will be especially advantageous
ταύτῃ καὶ τὰς συνθήκας ποιησώμεθα,
by means of those of the terms they should make
οὕτω κατά γε τὸ εἰκὸς μάλιστα συμμένοιμεν ἄν.
thus according to the most probable we would especially be able to keep (the agreement) ...