Deuteronomy 21:13

10 “When you go out to war against your enemies, and the Lord your God gives them into your hand and you take them captive, 11 and you see among the captives a beautiful woman, and you desire to take her to be your wife, 12 and you bring her home to your house, she shall shave her head and pare her nails. 13 And she shall take off the clothes in which she was captured and shall remain in your house and lament her father and her mother a full month. After that you may go in to her and be her husband, and she shall be your wife. 14 But if you no longer delight in her, you shall let her go where she wants. But you shall not sell her for money, nor shall you treat her as a slave, since you have humiliated her.

15 “If a man has two wives, the one loved and the other unloved, and both the loved and the unloved have borne him children, and if the firstborn son belongs to the unloved, 16 then on the day when he assigns his possessions as an inheritance to his sons, he may not treat the son of the loved as the firstborn in preference to the son of the unloved, who is the firstborn,