Deuteronomy 7:8

7:8 Rather it is because of his love for you and his faithfulness to the promise he solemnly vowed to your ancestors that the Lord brought you out with great power, redeeming you from the place of slavery, from the power of Pharaoh king of Egypt.

Deuteronomy 21:17

21:17 Rather, he must acknowledge the son of the less loved wife as firstborn and give him the double portion of all he has, for that son is the beginning of his father’s procreative power – to him should go the right of the firstborn.