Deuteronomy 21:14-17

14And it shall be, if thou have no delight in her, then thou shalt let her go according to her desire; but thou shalt in no wise sell her for money; thou shalt not treat her as a slave, because thou hast humbled her.

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;

17but he shall acknowledge as firstborn the son of the hated, by giving him a double portion of all that is found with him; for he is the firstfruits of his vigour: the right of the firstborn is his.