Passage 9.29.4
εἰσὶ δʼ οἳ καὶ αὐτῷ θυγατέρας ἐννέα Πιέρῳ γενέσθαι λέγουσι καὶ τὰ ὀνόματα ἅπερ ταῖς θεαῖς τεθῆναι καὶ ταύταις, καὶ ὅσοι Μουσῶν παῖδες ἐκλήθησαν ὑπὸ Ἑλλήνων, θυγατριδοῦς εἶναι σφᾶς Πιέρου· Μίμνερμος δέ, ἐλεγεῖα ἐς τὴν μάχην ποιήσας τὴν Σμυρναίων πρὸς Γύγην τε καὶ Λυδούς, φησὶν ἐν τῷ προοιμίῳ θυγατέρας Οὐρανοῦ τὰς ἀρχαιοτέρας Μούσας, τούτων δὲ ἄλλας νεωτέρας εἶναι Διὸς παῖδας.
There are some who say that Pierus himself also had nine daughters, and that they received the very same names that had been given to the goddesses; thus, all whom the Greeks called children of the Muses would actually be descendants of Pierus’s daughters. But Mimnermus, in the elegies he composed about the battle of the Smyrnaeans against Gyges and the Lydians, states in his prologue that the elder Muses were daughters of Uranus, and that beside these there are other, younger ones, who are daughters of Zeus.