Passage 9.15.1
μετὰ δὲ ταῦτα ἐν Θεσσαλίᾳ δυναστεύων Ἀλέξανδρος Πελοπίδαν ἐλθόντα---ἀφίκετο δὲ ὡς παρὰ ἄνδρα ἰδίᾳ τε εὔνουν αὑτῷ καὶ Θηβαίων φίλον τῷ κοινῷ--- παραγενόμενον δὴ τὸν Πελοπίδαν ὁ Ἀλέξανδρος δήσας εἶχεν ὑπὸ ἀπιστίας τε καὶ ὕβρεως· Θηβαῖοι δὲ ἐπεξιέναι παραυτίκα ἐπὶ τὸν Ἀλέξανδρον ὥρμηντο. ἡγεμόνας μὲν οὖν τῆς ἐξόδου Κλεομένην καὶ Ὕπατον ἐποιήσαντο βοιωταρχοῦντας ἐν τῷ τότε· Ἐπαμινώνδᾳ δὲ συνέβαινεν ἐν τοῖς στρατευομένοις τετάχθαι.
Afterwards, Alexander, who held power in Thessaly, arrested Pelopidas when he came as if to a man who was personally well-disposed towards him and friendly to the Thebans as a people in general. Indeed, upon Pelopidas' arrival, Alexander, prompted by mistrust and insolence, immediately seized and imprisoned him. The Thebans straightaway resolved to march forth against Alexander. They appointed Cleomenes and Hypatus, then serving as Boeotarchs, as leaders of the expedition. Epaminondas also happened to be numbered among those who marched out.