Passage 6.20.5
ἐπῄεσάν τε δὴ οἱ Ἀρκάδες καὶ τὸ παιδίον ἐνταῦθα ἤδη δράκων ἦν· ταραχθεῖσι δὲ ἐπὶ τῷ θεάματι τοῖς Ἀρκάσι καὶ ἐνδοῦσιν ἐς φυγὴν ἐπέκειντο οἱ Ἠλεῖοι, καὶ νίκην τε ἐπιφανεστάτην ἀνείλοντο καὶ ὄνομα τῷ θεῷ τίθενται Σωσίπολιν. ἔνθα δέ σφισιν ὁ δράκων ἔδοξεν ἐσδῦναι μετὰ τὴν μάχην, τὸ ἱερὸν ἐποίησαν ἐνταῦθα· σὺν δὲ αὐτῷ σέβεσθαι καὶ τὴν Εἰλείθυιαν ἐνόμισαν, ὅτι τὸν παῖδά σφισιν ἡ θεὸς αὕτη προήγαγεν ἐς ἀνθρώπους.
The Arcadians then advanced, and at that very moment the child was transformed into a serpent. Disturbed by this sudden vision, the Arcadians fell into confusion and took to flight; the Eleans pressed upon them, winning a most conspicuous victory. Thereafter, they named their deity Sosipolis ("Savior of the City"). On the spot where they believed the serpent had concealed itself after the battle, they built a temple. It became their custom also to revere Eileithyia in association with this deity, believing that it was this goddess who had brought forth the child among mankind.