ὤδινεν ὄρος, Ζεὺς δ' ἐφοβεῖτο, τὸ δ' ἔτεκεν μῦν -> the mountain was in labor — even Zeus was afraid — but gave birth to a mouse (Theopompus, Sotades, etc.).
Cf. Galen, Corpus medicorum Graecorum, vol. 5.9: "But that story is like the old saying we learned in childhood, 'The mountain labored but gave birth to a mouse (ὤδινεν ὄρος καὶ ἔτεκε μῦν)'."