Deuteronomy 21:15&16

15 "If a man has two wives, the one loved and the other disliked, and they have borne him children, both the loved and the disliked, and if the first-born son is hers that is disliked, 16 then on the day when he assigns his possessions as an inheritance to his sons, he may not treat the son of the loved as the first-born in preference to the son of the disliked, who is the first-born.