Diodorus Siculus, Library of History (1.96):
The priests of the Egyptians relate from the registers in their sacred books that long ago, they were visited by Orpheus and Musaeus, Melampus and Daedalus, and in addition to these by Homer the poet and Lycurgus the Spartan, and even by Solon of Athens and Plato the philosopher; Pythagoras of Samos came to Egypt too, as did the mathematician Eudoxus, Democritus of Abdera, and Oenopides of Chios. As signs of all this, they show images of them, they demonstrate their names applied to various places and structures, and they produce demonstrations of their enviable learning, affirming that everything that people marvel at in the Greeks actually came from Egypt.
They claim that most of the mystic rites of Orpheus, the celebrations related to his wandering, and the stories about his time in Hades came from there. They say that the rites of Osiris are the same as those of Dionysus, those of Isis more or less the same as those of Demeter, with the names alone being different. The punishments of the wicked in Hades, the fields of the blessed, and the fashioning of images produce something imitating burials as they are performed in Egypt. They hold that Hermes the Psychompompus, in accordance with ancient Egyptian custom, leads the body of Apis back from the dead and gives it to someone wearing the face of Cerberus. After Orpheus made this clear to the Greeks, Homer followed him in setting it down in verse thus:
Cyllenian Hermes called forth the souls of the suitors, holding the staff in his hand.
Then, as he descended again,
They went by the waves of the Ocean and the White Rock,
Past the gates of Helios and the land of Dreams;
Suddenly they came to the Grove of Asphodel
Where souls live, the phantoms of exhausted mortals.
οἱ γὰρ ἱερεῖς τῶν Αἰγυπτίων ἱστοροῦσιν ἐκ τῶν ἀναγραφῶν τῶν ἐν ταῖς ἱεραῖς βίβλοις παραβαλεῖν πρὸς ἑαυτοὺς τὸ παλαιὸν Ὀρφέα τε καὶ Μουσαῖον καὶ Μελάμποδα καὶ Δαίδαλον, πρὸς δὲ τούτοις Ὅμηρόν τε τὸν ποιητὴν καὶ Λυκοῦργον τὸν Σπαρτιάτην, ἔτι δὲ Σόλωνα τὸν Ἀθηναῖον καὶ Πλάτωνα τὸν φιλόσοφον, ἐλθεῖν δὲ καὶ Πυθαγόραν τὸν Σάμιον καὶ τὸν μαθηματικὸν Εὔδοξον, ἔτι δὲ Δημόκριτον τὸν Ἀβδηρίτην καὶ Οἰνοπίδην τὸν Χῖον. πάντων δὲ τούτων σημεῖα δεικνύουσι τῶν μὲν εἰκόνας, τῶν δὲ τόπων ἢ κατασκευασμάτων ὁμωνύμους προσηγορίας, ἔκ τε τῆς ἑκάστῳ ζηλωθείσης παιδείας ἀποδείξεις φέρουσι, συνιστάντες ἐξ Αἰγύπτου μετενηνέχθαι πάντα δι᾽ ὧν παρὰ τοῖς Ἕλλησιν ἐθαυμάσθησαν.
Ὀρφέα μὲν γὰρ τῶν μυστικῶν τελετῶν τὰ πλεῖστα καὶ τὰ περὶ τὴν ἑαυτοῦ πλάνην ὀργιαζόμενα καὶ τὴν τῶν ἐν ᾄδου μυθοποιίαν ἀπενέγκασθαι. τὴν μὲν γὰρ Ὀσίριδος τελετὴν τῇ Διονύσου τὴν αὐτὴν εἶναι, τὴν δὲ τῆς Ἴσιδος τῇ τῆς Δήμητρος ὁμοιοτάτην ὑπάρχειν, τῶν ὀνομάτων μόνων ἐνηλλαγμένων: τὰς δὲ τῶν ἀσεβῶν ἐν ᾄδου τιμωρίας καὶ τοὺς τῶν εὐσεβῶν λειμῶνας καὶ τὰς παρὰ τοῖς πολλοῖς εἰδωλοποιίας ἀναπεπλασμένας παρεισαγαγεῖν μιμησάμενον τὰ γινόμενα περὶ τὰς ταφὰς τὰς κατ᾽ Αἴγυπτον. τὸν μὲν γὰρ ψυχοπομπὸν Ἑρμῆν κατὰ τὸ παλαιὸν νόμιμον παρ᾽ Αἰγυπτίοις ἀναγαγόντα τὸ τοῦ Ἄπιδος σῶμα μέχρι τινὸς παραδιδόναι τῷ περικειμένῳ τὴν τοῦ Κερβέρου προτομήν. τοῦ δ᾽ Ὀρφέως τοῦτο καταδείξαντος παρὰ τοῖς Ἕλλησι τὸν Ὅμηρον ἀκολούθως τούτῳ θεῖναι κατὰ τὴν ποίησιν
Ἑρμῆς δὲ ψυχὰς Κυλλήνιος ἐξεκαλεῖτο
ἀνδρῶν μνηστήρων, ἔχε δὲ ῥάβδον μετὰ χερσίν.
εἶτα πάλιν ὑποβάντα λέγειν
πὰρ δ᾽ ἴσαν Ὠκεανοῦ τε ῥοὰς καὶ Λευκάδα πέτρην,
ἠδὲ παρ᾽ Ἠελίοιο πύλας καὶ δῆμον Ὀνείρων
ἤισαν: αἶψα δ᾽ ἵκοντο κατ᾽ ἀσφοδελὸν λειμῶνα,
ἔνθα τε ναίουσι ψυχαί, εἴδωλα καμόντων