Deuteronomy 21:15-17

American Standard Version

¹⁵ If a man have two wives, the one beloved, and the other hated, and they have borne him children, both the beloved and the hated; and if the first-born son be hers that was hated; ¹⁶ then it shall be, in the day that he causeth his sons to inherit that which he hath, that he may not make the son of the beloved the first-born ^[a]before the son of the hated, who is the first-born: ¹⁷ but he shall acknowledge the first-born, the son of the hated, by giving him a double portion of all that he hath; for he is the beginning of his strength; the right of the first-born is his.