Deuteronomy 21:15-17

A man might have two wives, one he loves and one he doesn’t. Both wives might have sons by him. If the older son belongs to the wife he does not love, when that man wills his property to his sons he must not give the son of the wife he loves what belongs to the older son, the son of the wife he does not love. He must agree to give the older son two shares of everything he owns, even though the older son is from the wife he does not love. That son was the first to prove his father could have children, so he has the rights that belong to the older son.