Passage 8.10.8
ἔλαθόν τε δὴ κατὰ νώτου γενόμενοί σφισιν οἱ Ἀρκάδες καὶ οἱ Λακεδαιμόνιοι κυκλωθέντες τῆς τε ἄλλης στρατιᾶς τὸ πολὺ ἀποβάλλουσι καὶ βασιλεὺς ἔπεσεν Ἆγις Εὐδαμίδου. φανῆναι δὲ καὶ τὸν Ποσειδῶνα ἀμύνοντά σφισιν ἔφασαν οἱ Μαντινεῖς, καὶ τοῦδε ἕνεκα τρόπαιον ἐποιήσαντο ἀνάθημα τῷ Ποσειδῶνι.
The Arcadians got behind the Lacedaemonians without their noticing, encircled them, and the Spartans consequently lost the greater part of the rest of their army, while King Agis, son of Eudamidas, fell. The Mantineans claimed that Poseidon himself had appeared, aiding them in battle, and for this reason they set up a trophy as an offering to Poseidon.