Passage 10.22.8
περὶ δὲ τοὺς Ἕλληνας ἐν τῷ αὐτῷ χρόνῳ τοὺς ἐν Θερμοπύλαις συνέβαινεν ἄλλα τοιαῦτα. ἀτραπός ἐστι διὰ τοῦ ὄρους τῆς Οἴτης, μία μὲν ἡ ὑπὲρ Τραχῖνος ἀπότομός τε τὰ πλείω καὶ ὄρθιος δεινῶς, ἑτέρα δὲ ἡ διὰ τῆς Αἰνιάνων ὁδεῦσαι στρατῷ ῥᾴων, διʼ ἧς καὶ Ὑδάρνης ποτὲ Μῆδος κατὰ νώτου τοῖς περὶ Λεωνίδην ἐπέθετο Ἕλλησι.
At the same time as this, other such events happened among the Greeks at Thermopylae. There is a path through Mount Oeta: one above Trachis, mostly precipitous and exceedingly steep; another through the land of the Aenianians, easier for an army to traverse. It was by this route that once the Mede Hydarnes attacked from behind the Greeks under Leonidas.