Passage 9.1.2
ὅτι μὲν δὴ καὶ οὗτοι τὸ ἀρχαῖον ἐβασιλεύοντο, δῆλά ἐστι· βασιλεῖαι γὰρ πανταχοῦ τῆς Ἑλλάδος καὶ οὐ δημοκρατίαι πάλαι καθεστήκεσαν. τῶν δὲ βασιλέων ἄλλον μὲν οὐδένα οἱ Πλαταιεῖς ἴσασι, μόνον δὲ Ἀσωπὸν καὶ ἔτι πρότερον Κιθαιρῶνα· καὶ τὸν μὲν ἀφʼ αὑτοῦ θέσθαι τῷ ὄρει τὸ ὄνομα, τὸν δὲ τῷ ποταμῷ λέγουσι. δοκῶ δὲ καὶ τὴν Πλάταιαν, ἀφʼ ἧς κέκληται καὶ ἡ πόλις, βασιλέως Ἀσωποῦ καὶ οὐ τοῦ ποταμοῦ παῖδα εἶναι.
That indeed the Plataeans were also anciently ruled by kings is clear; for in every region of Greece kingdoms and not democracies were formerly established. But of their kings the Plataeans know none apart from Asopus, and earlier still Cithaeron; they say that the latter gave his own name to the mountain and the former to the river. In my opinion, Plataea as well, from whom the city derives its name, was a daughter of King Asopus and not of the river.