Passage 9.15.2
γεγονυίας δὲ ἐκτὸς Πυλῶν οἱ τῆς δυνάμεως ἐπιτίθεταί σφισιν ἐν δυσχωρίαις λοχήσας ὁ Ἀλέξανδρος· ὡς δὲ ἐφαίνετο ἄπορα εἶναι τὰ τῆς σωτηρίας, οὕτω δὴ ὅ τε λοιπὸς στρατὸς ἡγεμόνα ποιοῦνται τὸν Ἐπαμινώνδαν καὶ οἱ Βοιωτάρχαι παραχωροῦσιν ἑκουσίως τῆς ἀρχῆς· Ἀλέξανδρος δὲ οὔτε ἔτι ἐθάρρει τὸν πόλεμον στρατηγοῦντα ὁρῶν τοῖς ἐναντίοις Ἐπαμινώνδαν καὶ ἑκὼν Πελοπίδαν ἀφίησιν.
After they had marched out beyond the Gates, Alexander, having set an ambush at a difficult pass, attacked them with his force. And when it appeared that their situation offered no hope of escape, the rest of the army chose Epaminondas as their leader, and the Boeotarchs willingly yielded command to him. Alexander, seeing Epaminondas now leading his opponents, no longer had the courage to continue the war, and he voluntarily released Pelopidas.