Passage 7.6.4
ὑστέρησαν δὲ καὶ ἔργου τοῦ Πλαταιᾶσι· δῆλα γὰρ δὴ ὅτι ἐπὶ τῷ ἀναθήματι τῷ ἐν Ὀλυμπίᾳ τῶν Ἑλλήνων μετῆν ἂν καὶ Ἀχαιοῖς γεγράφθαι. δοκεῖν δέ μοι τὰς πατρίδας τε ὑπολειφθέντες ἕκαστοι τὰς αὑτῶν ἔσωζον καὶ ἅμα διὰ τὸ ἔργον τὸ πρὸς Τροίαν Λακεδαιμονίους Δωριεῖς ἀπηξίουν σφίσιν ἡγεῖσθαι. ἐδήλωσαν δὲ καὶ ἀνὰ χρόνον· Λακεδαιμονίων γὰρ ἐς τὸν πρὸς Ἀθηναίους πόλεμον καταστάντων ὕστερον, ἐς τὴν συμμαχίαν ἦσαν οἱ Ἀχαιοὶ πρόθυμοι Πατρεῦσι, καὶ ἐς τοὺς Ἀθηναίους οὐχ ἧσσον εἶχον γνώμην.
They were also absent from the military action at Plataea; indeed, it is evident that if they had been present, the Achaeans too would certainly have been inscribed on the Greek offering at Olympia. I suspect that they each remained behind to guard their own territory, and moreover, considering the experience of the Trojan War, they regarded the Lacedaemonians—the Dorians—as unworthy to command them. They clearly demonstrated this attitude again in later times. For afterwards, when the Lacedaemonians found themselves at war with the Athenians, the Achaeans willingly allied themselves with the people of Patrae and were no less favorably inclined toward the Athenians.