Charles Ellicott Commentary Genesis 31:42

"Except the God of my father, the God of Abraham, and the Fear of Isaac, had been with me, surely now hadst thou sent me away empty. God hath seen mine affliction and the labor of my hands, and rebuked thee yesternight." — Genesis 31:42 (ASV)

The fear of Isaac—That is, the object of Isaac’s worship. The reason given by Jewish Commentators for this remarkable way of describing the Deity whom Isaac served is that because his father was still alive, Jacob would have been lacking in reverence if he had spoken of God as “Isaac’s God,” even though Jehovah had condescended to call Himself so (Genesis 28:13).