Deut 21:15-17

The Right of the Firstborn 15If a man has two wives, one of them loved and the other disliked, and if both the loved and the disliked have borne him sons, the firstborn being the son of the one who is disliked,16then on the day when he wills his possessions to his sons, he is not permitted to treat the son of the loved as the firstborn in preference to the son of the disliked, who is the firstborn.17He must acknowledge as firstborn the son of the one who is disliked, giving him a double portion of all that he has; since he is the first issue of his virility, the right of the firstborn is his.