Passage 2.1.6
τὸ δὲ οὐ Κορινθίοις μόνον περὶ τῆς χώρας ἐστὶν εἰρημένον, ἀλλὰ ἐμοὶ δοκεῖν Ἀθηναῖοι πρῶτοι περὶ τῆς Ἀττικῆς ἐσεμνολόγησαν· λέγουσι δὲ καὶ οἱ Κορίνθιοι Ποσειδῶνα ἐλθεῖν Ἡλίῳ περὶ τῆς γῆς ἐς ἀμφισβήτησιν, Βριάρεων δὲ διαλλακτὴν γενέσθαι σφίσιν, ἰσθμὸν μὲν καὶ ὅσα ταύτῃ δικάσαντα εἶναι Ποσειδῶνος, τὴν δὲ ἄκραν Ἡλίῳ δόντα τὴν ὑπὲρ τῆς πόλεως. ἀπὸ μὲν τούτου λέγουσιν εἶναι τὸν ἰσθμὸν Ποσειδῶνος·
This boastful claim concerning their land is not unique to the Corinthians; rather, it seems to me the Athenians were the first to extol Attica in such a manner. As for the Corinthians, they also say that Poseidon disputed with Helios over their territory, and that Briareus acted as arbitrator between them, assigning the Isthmus and the territory surrounding it to Poseidon, while granting to Helios the promontory that rises above the city. They say this is why the Isthmus belongs to Poseidon.