Deuteronomy 21:15-17

If a man has two wives, and he loves one but not the other, and both bear him sons but the firstborn is the son of the wife he does not love, when he wills his property to his sons, he must not give the rights of the firstborn to the son of the wife he loves in preference to his actual firstborn, the son of the wife he does not love. He must acknowledge the son of his unloved wife as the firstborn by giving him a double share of all he has. That son is the first sign of his father’s strength. The right of the firstborn belongs to him.