Deuteronomy 21:15-16

15If a man have two wives, one beloved, and one hated, and they have borne him children, both the beloved and the hated, and the firstborn son be hers that was hated;

16then it shall be, in the day that he maketh his sons to inherit what he hath, that he may not make the son of the beloved firstborn before the son of the hated, who is the firstborn;