זוהר שמות — דף קיא ע”ב

The serpent stands at the door like a dog, and when the woman's voice issues forth, the Holy One sends a key and the serpent follows the sound to the mountain of the womb. Jacob wrestled with the minister of Esav and his thigh was weakened; Samuel then came and wrested the thigh from him so he would have no share in holiness. The Holy One gave the minister of Esav the belly and thigh of the sotah in exchange for the thigh that had been taken from him.