Deuteronomy 21
16 Ref 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:
17 Ref 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.