Passage 9.8.4
Θηβαίοις δὲ ἐν τῷ περιβόλῳ τοῦ ἀρχαίου τείχους ἑπτὰ ἀριθμὸν ἦσαν πύλαι, μένουσι δὲ καὶ ἐς ἡμᾶς ἔτι. τεθῆναι δὲ τὰ ὀνόματα ἐπυνθανόμην σφίσιν ἀπό τε Ἠλέκτρας ἀδελφῆς Κάδμου καὶ Προιτίσιν ἀπὸ ἀνδρὸς τῶν ἐπιχωρίων· ἡλικίαν δὲ Προίτου καὶ τὸ ἀνωτέρω γένος χαλεπὰ ἦν εὑρεῖν. τὰς δὲ Νηίστας ὀνομασθῆναί φασιν ἐπὶ τῷδε. ἐν ταῖς χορδαῖς νήτην καλοῦσι τὴν ἐσχάτην· ταύτην οὖν τὴν χορδὴν Ἀμφίονα ἐπὶ ταῖς πύλαις ταύταις ἀνευρεῖν λέγουσιν. ἤδη δὲ ἤκουσα καὶ ὡς Ζήθου τοῦ ἀδελφοῦ τοῦ Ἀμφίονος τῷ παιδὶ ὄνομα Νῆις γένοιτο, ἀπὸ τούτου δὲ τοῦ Νήϊδος τὰς πύλας κληθῆναι ταύτας.
For the Thebans, within the circuit of the ancient wall, there were seven gates in number, and they remain even to our own time. As I learned, their names were derived for them partly from Electra, the sister of Cadmus, and partly from Proetus, a local man. But it proved difficult to discover the epoch of Proetus or the facts about his earlier lineage. The Neistan gates, they say, got their name for the following reason: among the musical strings, they call the lowest one "Neate." This particular string, then, they claim Amphion discovered at these gates. But I have also heard another account, that Zethus, Amphion's brother, had a son named Neis, and that these gates were named Neistan after this Neis.