Genesis 25:23

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. The answer was thus given in rhythmic parallels, the form of Hebrew poetry, stating that the older of the twins, the first-born, would be the servant of the younger. Cf Romans 9:11. The younger, therefore, would be the bearer and heir of the Messianic promise, the spiritual fatherhood of Abraham should be continued in him.

the sons of Leah: Reuben, Jacob's first-born, and Simeon, and Levi, and Judah, and Issachar, and Zebulun;