Passage 6.26.9
ἤκουσα δὲ καὶ ὡς οὐχ ἡ Ἐρυθρά, ποταμὸς δὲ ὃν Σῆρα ὀνομάζουσιν, οὗτός ἐστιν ὁ ποιῶν νῆσον αὐτήν, ὥσπερ καὶ Αἰγύπτου τὸ Δέλτα ὑπὸ τοῦ Νείλου καὶ οὐχ ὑπὸ μιᾶς περιέχεσθαι θαλάσσης· τοιαύτην ἑτέραν καὶ τὴν Σηρίαν νῆσον εἶναι. οὗτοι μὲν δὴ τοῦ Αἰθιόπων γένους αὐτοί τέ εἰσιν οἱ Σῆρες καὶ ὅσοι τὰς προσεχεῖς αὐτῇ νέμονται νήσους, Ἄβασαν καὶ Σακαίαν· οἱ δὲ αὐτοὺς οὐκ Αἰθίοπας, Σκύθας δὲ ἀναμεμιγμένους Ἰνδοῖς φασὶν εἶναι.
I have also heard it said that Erythra is not actually the sea, but a river called Sera, and that it is this river which makes it into an island, just as Egypt's Delta is formed by the Nile, being surrounded not purely by sea. They say another island, similarly formed, is that called Seria. These Seres themselves are indeed of the Ethiopian race, as are those who inhabit the nearby islands of Abasa and Sacaea. But others assert that they are not Ethiopians, but rather Scythians intermingled with Indians.