Passage 5.14.2
τῆς δὲ λεύκης μόνης τοῖς ξύλοις ἐς τοῦ Διὸς τὰς θυσίας καὶ ἀπʼ οὐδενὸς δένδρου τῶν ἄλλων οἱ Ἠλεῖοι χρῆσθαι νομίζουσι, κατʼ ἄλλο μὲν οὐδὲν προτιμῶντες ἐμοὶ δοκεῖν τὴν λεύκην, ὅτι δὲ Ἡρακλῆς ἐκόμισεν αὐτὴν ἐς Ἕλληνας ἐκ τῆς Θεσπρωτίδος χώρας. καί μοι καὶ αὐτὸς ὁ Ἡρακλῆς ἐφαίνετο, ἡνίκα τῷ Διὶ ἔθυεν ἐν Ὀλυμπίᾳ, τῶν ἱερείων τὰ μηρία ἐπὶ λεύκης καῦσαι ξύλων· τὴν δὲ λεύκην ὁ Ἡρακλῆς πεφυκυῖαν παρὰ τὸν Ἀχέροντα εὗρε τὸν ἐν Θεσπρωτίᾳ ποταμόν, καὶ τοῦδε ἕνεκά φασιν αὐτὴν Ἀχερωίδα ὑπὸ Ὁμήρου καλεῖσθαι.
Only the wood of the white poplar do the Eleans regard as appropriate for sacrifices to Zeus, making use of no other tree whatsoever. To my thinking, their preference for the white poplar is based not on any quality peculiar to the tree itself but because Heracles brought it to Greece from the land of Thesprotia. It seemed to me that Heracles himself employed its wood when sacrificing to Zeus at Olympia, burning upon white poplar wood the thighs of the sacrificial victims. Heracles found this tree growing beside the river Acheron in Thesprotia; and for this reason, they say, Homer calls it "Acherōïs."