Genesis 25:20-24

And Isaac was forty years old when he took Rebekah to wife, the daughter of Bethuel the Syrian of Padan-aram, the sister to Laban the Syrian.

And Isaac intreated the LORD for his wife, because she was barren: and the LORD was intreated of him, and Rebekah his wife conceived.

And the children struggled together within her; and she said, If it be so, why am I thus? And she went to enquire of the LORD.

And the LORD said unto her, Two nations are in thy womb, and two manner of people shall be separated from thy bowels; and the one people shall be stronger than the other people; and the elder shall serve the younger.

And when her days to be delivered were fulfilled, behold, there were twins in her womb.