Deuteronomy 21:15-17

If a man has two wives, one greatly loved and the other hated, and the two of them have had children by him; and if the first son is the child of the hated wife: Then when he gives his property to his sons for their heritage, he is not to put the son of his loved one in the place of the first son, the son of the hated wife: But he is to give his first son his birthright, and twice as great a part of his property: for he is the first-fruits of his strength and the right of the first son is his.