Passage 1.19.5
ταῦτα μὲν οὕτω γενέσθαι λέγουσι· ποταμοὶ δὲ Ἀθηναίοις ῥέουσιν Ἰλισός τε καὶ Ἠριδανῷ τῷ Κελτικῷ κατὰ τὰ αὐτὰ ὄνομα ἔχων, ἐκδιδοὺς ἐς τὸν Ἰλισόν. ὁ δὲ Ἰλισός ἐστιν οὗτος, ἔνθα παίζουσαν Ὠρείθυιαν ὑπὸ ἀνέμου Βορέου φασὶν ἁρπασθῆναι· καὶ συνοικεῖν Ὠρειθυίᾳ Βορέαν καί σφισι διὰ τὸ κῆδος ἀμύναντα τῶν τριήρων τῶν βαρβαρικῶν ἀπολέσαι τὰς πολλάς. ἐθέλουσι δὲ Ἀθηναῖοι καὶ ἄλλων θεῶν ἱερὸν εἶναι τὸν Ἰλισόν, καὶ Μουσῶν βωμὸς ἐπʼ αὐτῷ ἐστιν Ἰλισιάδων· δείκνυται δὲ καὶ ἔνθα Πελοποννήσιοι Κόδρον τὸν Μελάνθου βασιλεύοντα Ἀθηναίων κτείνουσι.
These things, then, are said to have happened in this way. There are rivers flowing through Athens, both the Ilissus and the Eridanus, the latter having the same name as the Celtic river and flowing into the Ilissus. It is by the Ilissus where, as they say, Oreithyia was playing when she was seized away by Boreas, the North Wind. Boreas lived with Oreithyia as his wife, and on account of this marriage, he aided the Athenians by destroying most of the barbarian fleet of triremes. The Athenians also make the Ilissus sacred to other gods, and upon it is an altar of the Muses known as the Ilissiads. It is also shown as the place where the Peloponnesians killed Codrus, son of Melanthus, who ruled the Athenians.