Passage 10.22.10
τότε δὲ ἡ τῶν Αἰνιάνων καὶ ἡ τῶν Ἡρακλεωτῶν ὑπόσχεσις ἐπήγειρε τὸν Βρέννον· καὶ Ἀκιχώριον μὲν κατέλιπεν ἐπὶ τῇ στρατιᾷ, προειπών, ἐπειδὰν περιλάβωσιν αὐτοὶ τὸ Ἑλληνικόν, τηνικαῦτα καὶ ἐκείνοις ἐφόδου καιρὸν εἶναι· ἀπολέξας δὲ αὐτὸς μυριάδας τοῦ στρατοῦ τέσσαρας ἐποιεῖτο τὴν ὁδὸν διὰ τῆς ἀτραποῦ.
Then the promise from the Aenianians and the Heracleots encouraged Brennus. He left Acichorius behind in charge of the army, instructing him that when they themselves had surrounded the Greeks, then it would be the proper time for them to make an attack. Selecting forty thousand from his force, he himself set out along the narrow mountain path.