Passage 6.25.2
ὁ δὲ ἱερὸς τοῦ Ἅιδου περίβολός τε καὶ ναός---ἔστι γὰρ δὴ Ἠλείοις καὶ Ἅιδου περίβολός τε καὶ ναός--- ἀνοίγνυται μὲν ἅπαξ κατὰ ἔτος ἕκαστον, ἐσελθεῖν δὲ οὐδὲ τότε ἐφεῖται πέρα γε τοῦ ἱερωμένου. ἀνθρώπων δὲ ὧν ἴσμεν μόνοι τιμῶσιν Ἅιδην Ἠλεῖοι κατὰ αἰτίαν τήνδε. Ἡρακλεῖ στρατιὰν ἄγοντι ἐπὶ Πύλον τὴν ἐν τῇ Ἤλιδι, παρεῖναί οἱ καὶ Ἀθηνᾶν συνεργὸν λέγουσιν· ἀφικέσθαι οὖν καὶ Πυλίοις τὸν Ἅιδην συμμαχήσοντα τῇ ἀπεχθείᾳ τοῦ Ἡρακλέους, ἔχοντα ἐν τῇ Πύλῳ τιμάς.
The sacred precinct and temple of Hades—for the Eleans indeed have both a precinct and temple dedicated to Hades—is opened once each year, but even then entrance is permitted to none except the appointed priest. The Eleans alone, of all the people known to us, worship Hades, and for the following reason: When Heracles was leading an army against the city of Pylos in Elis, Athena, it is said, came to his aid. Then, out of hatred for Heracles, Hades himself arrived to assist the Pylians, owing to the honors he held in Pylos.