Deuteronomy 21:14-17

14And it shall be, if thou have no delight in her, then thou shalt let her go whither she will; but thou shalt not sell her at all for money, thou shalt not deal with her as a slave, because thou hast humbled her.

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

16then it shall be, in the day that he causeth his sons to inherit that which he hath, that he may not make the son of the beloved the firstborn before the son of the hated, which is the firstborn:

17but he shall acknowledge the firstborn, the son of the hated, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his.