Passage 2.38.3
οὗτος μὲν δή σφισιν ἐκ τελετῆς, ἣν ἄγουσι τῇ Ἥρᾳ, λόγος τῶν ἀπορρήτων ἐστίν· τὰ δὲ ὑπὸ τῶν ἐν Ναυπλίᾳ λεγόμενα ἐς τὸν ὄνον, ὡς ἐπιφαγὼν ἀμπέλου κλῆμα ἀφθονώτερον ἐς τὸ μέλλον ἀπέφηνε τὸν καρπόν---καὶ ὄνος σφίσιν ἐν πέτρᾳ πεποιημένος διὰ τοῦτό ἐστιν ἅτε ἀμπέλων διδάξας τομήν---, παρίημι οὐκ ἀξιόλογα ἡγούμενος.
This indeed is the account of secret matters from the ritual they perform in honor of Hera. But the story told by the people of Nauplia concerning the donkey—that by nibbling excessively on a vine-shoot it revealed for future times a more abundant crop (and for this reason they have a donkey made of stone, commemorating how it taught them vine-pruning)—this story I pass over, considering it not worthy of mention.