Deuteronomy 21:15-16

New International Version

The Right of the Firstborn

¹⁵ If a man has two wives,^(A) and he loves one but not the other, and both bear him sons but the firstborn is the son of the wife he does not love,^{(B) 16} when he wills his property to his sons, he must not give the rights of the firstborn to the son of the wife he loves in preference to his actual firstborn, the son of the wife he does not love.^(C)