Passage 3.25.5
ἐποίησαν δὲ Ἑλλήνων τινὲς ὡς Ἡρακλῆς ἀναγάγοι ταύτῃ τοῦ Ἅιδου τὸν κύνα, οὔτε ὑπὸ γῆν ὁδοῦ διὰ τοῦ σπηλαίου φερούσης οὔτε ἕτοιμον ὂν πεισθῆναι θεῶν ὑπόγαιον εἶναί τινα οἴκησιν ἐς ἣν ἀθροίζεσθαι τὰς ψυχάς. ἀλλὰ Ἑκαταῖος μὲν ὁ Μιλήσιος λόγον εὗρεν εἰκότα, ὄφιν φήσας ἐπὶ Ταινάρῳ τραφῆναι δεινόν, κληθῆναι δὲ Ἅιδου κύνα, ὅτι ἔδει τὸν δηχθέντα τεθνάναι παραυτίκα ὑπὸ τοῦ ἰοῦ, καὶ τοῦτον ἔφη τὸν ὄφιν ὑπὸ Ἡρακλέους ἀχθῆναι παρʼ Εὐρυσθέα·
Some of the Greeks have made the story that Heracles led the hound of Hades up through this place, disregarding neither the idea that there might be a path underground through the cave nor the belief, which they were unwilling to accept, that beneath the earth existed some abode of gods where souls gathered together. But Hecataeus of Miletus provided a more plausible account, saying that a terrible serpent was reared at Taenarum, called the hound of Hades because anyone bitten immediately died from its venom. It was this serpent, he claimed, that Heracles brought to Eurystheus.