Passage 6.25.3
ἐπάγονται δὲ καὶ Ὅμηρον τῷ λόγῳ μάρτυρα ποιήσαντα ἐν Ἰλιάδι τλῆ δʼ Ἀίδης ἐν τοῖσι πελώριος ὠκὺν ὀιστόν, εὖτέ μιν ωὐτὸς ἀνὴρ υἱὸς Διὸς αἰγιόχοιο ἐν Πύλῳ ἐν νεκύεσσι βαλὼν ὀδύνῃσιν ἔδωκεν· Hom. Il. 5.395-397 εἰ δὲ κατὰ τὴν Ἀγαμέμνονος καὶ Μενελάου στρατείαν ἐπὶ Ἴλιον Ποσειδῶν τῷ Ὁμήρου λόγῳ τοῖς Ἕλλησιν ἐπίκουρος ἦν, οὐκ ἂν ἄπο τοῦ εἰκότος οὐδὲ Ἅιδην εἴη δόξῃ γε τοῦ αὐτοῦ ποιητοῦ Πυλίοις ἀμῦναι. Ἠλεῖοι δʼ οὖν ὡς σφίσι τε εὔνῳ καὶ ἀπεχθανομένῳ πρὸς τὸν Ἡρακλέα ἐποιήσαντο τὸ ἱερὸν τῷ θεῷ· ἑκάστου δὲ ἅπαξ ἀνοίγειν τοῦ ἐνιαυτοῦ νομίζουσιν, ὅτι οἶμαι καὶ ἀνθρώποις ἅπαξ ἡ κάθοδος ἡ ἐς τοῦ Ἅιδου γίνεται.
They also cite Homer as a witness to this story, since in the Iliad he says: "And mighty Hades endured among them a swift arrow, When the same man, the son of Zeus who bears the aegis, Struck him at Pylos among the shades, and gave him over to pains." (Hom. Il. 5.395-397) If then, during the expedition of Agamemnon and Menelaus against Ilium, Poseidon, according to Homer’s narrative, stood by the Greeks as an ally, it should not seem unreasonable for Hades too, according to the same poet, to have aided the Pylians. Thus, the Eleans made this sanctuary for Hades, considering that the god was friendly to themselves and hostile toward Heracles. They think it proper to open it only once each year, because, I suppose, men too descend to the realm of Hades only once.