Passage 8.51.1
Φιλοποίμην δὲ ὑπὸ τὸν καιρὸν ἐσπεσὼν τοῦτον ἐς τὴν Σπάρτην ἠνάγκασεν ἐς τὸ Ἀχαϊκὸν Λακεδαιμονίους συντελέσαι. μετὰ δὲ οὐ πολὺν χρόνον Τίτος μὲν Ῥωμαίων τῶν περὶ τὴν Ἑλλάδα ἡγεμὼν καὶ Διοφάνης ὁ Διαίου Μεγαλοπολίτης, ἄρχειν ἐν τῷ τότε ᾑρημένος τῶν Ἀχαιῶν, ἤλαυνον ἐπὶ τὴν Λακεδαίμονα, ἐπενεγκόντες αἰτίαν Λακεδαιμονίοις βουλεύειν σφᾶς νεώτερα ἐς Ῥωμαίους· Φιλοποίμην δέ, καίπερ ἐν τῷ παρόντι ἰδιώτης ὤν, ἀπέκλεισαν ὅμως ἐπιοῦσιν αὐτοῖς τὰς πύλας.
At that time Philopoemen made a sudden incursion into Sparta and compelled the Lacedaemonians to join the Achaean League. Not long afterwards, Titus, the commander of the Romans in Greece, and Diophanes son of Diaeus, of Megalopolis, who had been chosen at that time to lead the Achaeans, marched against Sparta, making the accusation that the Lacedaemonians were plotting fresh troubles against Rome. But Philopoemen, although at that moment he was a private citizen, nonetheless shut the gates against their advancing forces.