Deuteronomy 21:15-16

15“If a man has two wives, one loved and the other unloved, and they have borne him children, both the loved and the unloved, and if the firstborn son is of her who is unloved,

16then it shall be, on the day he bequeaths his possessions to his sons, that he must not bestow firstborn status on the son of the loved wife in preference to the son of the unloved, the true firstborn.