Deuteronomy 21:15-17

¹⁵"If a man has two wives, the one loved and the other disliked, and they have borne him children, both the loved and the disliked, and if the first-born son is hers that is disliked, ¹⁶then 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 first-born in preference to the son of the disliked, who is the first-born, ¹⁷but he shall acknowledge the first-born, the son of the disliked, by giving him a double portion of all that he has, for he is the first issue of his strength; the right of the first-born is his.