Deuteronomy 21:16; Deuteronomy 21:17

16 then it shall be in the day he wills what he has to his sons, he cannot make the son of the loved the firstborn before the son of the unloved, who is the firstborn.

17 "But he shall acknowledge the firstborn, the son of the unloved, by giving him a double portion of all that he has, for he is the beginning of his strength; to him belongs the right of the firstborn.