Deuteronomy 21:14

11 and see among the captives a beautiful woman, and you have a desire to her, and would take her to you as wife;

12 then you shall bring her home to your house; and she shall shave her head, and pare her nails;

13 and she shall put the clothing of her captivity from off her, and shall remain in your house, and bewail her father and her mother a full month: and after that you shall go in to her, and be her husband, and she shall be your wife.

14 It shall be, if you have no delight in her, then you shall let her go where she will; but you shall not sell her at all for money, you shall not deal with her as a slave, because you have humbled her.

15 If 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 who was hated;

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

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