Passage 1.30.3
Ἀκαδημίας δὲ οὐ πόρρω Πλάτωνος μνῆμά ἐστιν, ᾧ προεσήμαινεν ὁ θεὸς ἄριστον τὰ ἐς φιλοσοφίαν ἔσεσθαι· προεσήμαινε δὲ οὕτω. Σωκράτης τῇ προτέρᾳ νυκτὶ ἢ Πλάτων ἔμελλεν ἔσεσθαί οἱ μαθητὴς ἐσπτῆναί οἱ κύκνον ἐς τὸν κόλπον εἶδεν ὄνειρον· ἔστι δὲ κύκνῳ τῷ ὄρνιθι μουσικῆς δόξα, ὅτι Λιγύων τῶν Ἠριδανοῦ πέραν ὑπὲρ γῆς τῆς Κελτικῆς Κύκνον ἄνδρα μουσικὸν γενέσθαι βασιλέα φασί, τελευτήσαντα δὲ Ἀπόλλωνος γνώμῃ μεταβαλεῖν λέγουσιν αὐτὸν ἐς τὸν ὄρνιθα. ἐγὼ δὲ βασιλεῦσαι μὲν πείθομαι Λίγυσιν ἄνδρα μουσικόν, γενέσθαι δέ μοι ἄπιστον ὄρνιθα ἀπʼ ἀνδρός.
Not far from the Academy is the tomb of Plato, whom the god revealed beforehand would excel in philosophy. The manner of this revelation was as follows: Socrates, during the night preceding the day Plato was about to become his student, dreamed that a swan flew into his bosom. Now, the swan among birds has a reputation connected with music, because they say that across the river Eridanus, in the land of the Celts, there was once a musical man named Cycnus who became a king among the Ligurians, and after his death, by Apollo's will, he was transformed into a bird. As for myself, I am persuaded that indeed a musical man did rule over the Ligurians, but I find it difficult to believe that a human could become a bird.