Genesis 31:49-53

49It was also called Mizpah,1 because Laban said, “May the LORD keep watch between you and me when we are absent from each other. 50If you mistreat my daughters or take other wives, although no one is with us, remember that God is a witness between you and me.”

51Laban also said to Jacob, “Here is the mound, and here is the pillar I have set up between you and me. 52This mound is a witness, and this pillar is a witness, that I will not go past this mound to harm you, and you will not go past this mound and pillar to harm me. 53May the God of Abraham and the God of Nahor, the God of their father, judge between us.”

So Jacob swore by the Fear of his father Isaac.