Deuteronomy 21:15
15 'When a man hath two wives, the one loved and the other hated, and they have borne to him sons (the loved one and the hated one), and the first-born son hath been to the hated one;
Young's Literal Translation is in the public domain.
Matthew Henry's Commentary on Deuteronomy 21:15
Commentary on Deuteronomy 21:15-17
(Read Deuteronomy 21:15-17)
This law restrains men from disinheriting their eldest sons without just cause. The principle in this case as to children, is still binding to parents; they must give children their right without partiality.