Deuteronomy 21:15-17

Complete Jewish Bible

¹⁵ “If a man has two wives, the one loved and the other unloved, and both the loved and unloved wives have borne him children, and if the firstborn son is the child of the unloved wife; ¹⁶ then, when it comes time for him to pass his inheritance on to his sons, he may not give the inheritance due the firstborn to the son of the loved wife in place of the son of the unloved one, who is in fact the firstborn. ¹⁷ No, he must acknowledge as firstborn the son of the unloved wife by giving him a double portion of everything he owns, for he is the firstfruits of his manhood, and the right of the firstborn is his.