Passage 4.22.5
ταῦτα εἰπόντος τῶν Ἀρκάδων ὅσον τριακόσιοι μετέχειν καὶ αὐτοὶ τοῦ τολμήματος ἤθελον. καὶ τότε μὲν ἐπεῖχον τῆς ἐξόδου, τὰ γὰρ ἱερὰ ἐγίνετο αὐτοῖς οὐ κατὰ γνώμην, τῇ δὲ ἐπιούσῃ τό τε ἀπόρρητον ἔγνωσαν σφῶν τοὺς Λακεδαιμονίους προπεπυσμένους καὶ αὐτοὶ δεύτερον ὑπὸ Ἀριστοκράτους προδεδομένοι· τὰ γὰρ τοῦ Ἀριστομένους βουλεύματα αὐτίκα ὁ Ἀριστοκράτης ἐγγράψας βιβλίῳ, καὶ τὸ βιβλίον ἐπιθεὶς τῶν οἰκετῶν ὃν ἠπίστατο ὄντα εὐνούστατον, παρὰ Ἀνάξανδρον ἀπέστελλεν ἐς Σπάρτην.
When he had said this, about three hundred of the Arcadians themselves also wished to partake in the daring attempt. At that time, however, they delayed their departure, for the sacrifices did not prove favorable to them. The following day, they realized that the Lacedaemonians had already learned of their secret plan, and that they had been betrayed yet again by Aristocrates. Aristocrates immediately had written down Aristomenes' intentions in a letter, and entrusting the letter to the most trustworthy of his servants, dispatched it to Anaxander at Sparta.