Passage 10.20.6
τοῖς δὲ ἐς Θερμοπύλας ἀθροισθεῖσιν Ἑλλήνων, ὡς ἐπύθοντο περί τε Μαγνησίαν καὶ γῆν τὴν Φθιῶτιν ὄντα ἤδη τῶν Γαλατῶν τὸν στρατόν, ἔδοξεν ἤδη ψιλούς τε ἐς χιλίους καὶ ἐς τὴν ἵππον ἀπολέξαντας ἀποστεῖλαι σφᾶς ἐπὶ τὸν Σπερχειόν, ἵνα μηδὲ τὸν ποταμὸν διαβῆναι τοῖς βαρβάροις ἄνευ ἀγῶνός τε καὶ κινδύνων ἐγγένηται. οἱ δὲ ἐλθόντες τὰς γεφύρας τε καταλύουσι καὶ αὐτοὶ παρὰ τὴν ὄχθην ἐστρατοπεδεύσαντο. ἦν δὲ οὐδὲ ὁ Βρέννος οὔτε πάντα ἀσύνετος οὔτε ἀπείρως εἶχεν ὡς ἄν τις βάρβαρος σοφίσματα ἐς πολεμίους ἐξευρεῖν.
When the Greeks who had gathered at Thermopylae learned that the Galatian army was already in Magnesia and Phthiotis, they resolved to select about a thousand light-armed troops and a detachment of cavalry to send them to the River Spercheios, so that the barbarians would not cross the river without fighting and encountering danger. Upon arriving, they destroyed the bridges and made their camp along the riverbank. As for Brennus, he was neither entirely foolish nor inexperienced, for a barbarian, in discovering wiles against his enemies.