Passage 8.49.5
γενομένης δὲ ἐν Σελλασίᾳ πρὸς Κλεομένην τε καὶ Λακεδαιμονίους μάχης, ἣν Ἀχαιοὶ καὶ Ἀρκάδες ἀπὸ τῶν πόλεων πασῶν, σὺν δέ σφισι καὶ Ἀντίγονος ἐμαχέσατο ἄγων ἐκ Μακεδονίας στρατιάν, ἐτέτακτο μὲν τηνικαῦτα ὁ Φιλοποίμην ἐν τοῖς ἱππεῦσιν· ἐπεὶ δὲ ἐν τῷ πεζῷ τοῦ ἔργου τὸ πλεῖστον ἑώρα ληψόμενον τὴν κρίσιν, ὁπλίτης ἑκὼν ἐγένετο, καὶ αὐτὸν λόγου κινδυνεύοντα ἀξίως τῶν τις ἐναντίων διʼ ἀμφοτέρων ἔπειρε τῶν μηρῶν.
When the battle at Sellasia took place against Cleomenes and the Lacedaemonians, in which the Achaeans with Arcadians from all their cities fought alongside Antigonus, who had brought an army from Macedonia, Philopoemen at that time was posted among the horsemen. However, when he saw that the decisive issue would largely depend upon the infantry, he voluntarily became a foot soldier. Fighting in a manner worthy of renown, he was wounded by one of his opponents, who drove a spear right through both his thighs.