Deuteronomy 21:15-17 - The Right Of The Firstborn
15 When a man shall have two wives - the one beloved and, the other hated, and they have borne him sons, both she that is beloved, and she that is hated, - and it shall be that the firstborn son belongeth to her that is hated, 16 then shall it be, that in the day when he maketh his sons to inherit that which he hath, he may not treat the son of the beloved, instead of the son of the hated, as the firstborn; 17 but the true firstborn the son of her that is hated, shall he treat as firstborn by giving him double out of all that is found to be his, - for, he, is the beginning of hi strength, his, is the right of the firstborn.