Deuteronomy 21:15-17

New Catholic Bible

¹⁵ Law of the Firstborn.^[a] If a man has two wives, and he loves one and dislikes the other, and they both bear him children, both the one who is loved and the one who is disliked, and the son of the one who is disliked is the firstborn, ¹⁶ then when he gives his inheritance to his sons, he is not to give the rights of the firstborn to the son of the woman he loves in preference to the true firstborn, the son of the woman he dislikes. ¹⁷ He must acknowledge the son of the woman whom he disliked as the firstborn, giving him a double portion of everything that he owned. That son is the first sign of his strength, and the right of firstborn belongs to him.