Deuteronomy 21:15-17

¹⁵If 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 firstborn son be hers who was hated; ¹⁶then it shall be, in the day that he causes his sons to inherit that which he has, that he may not make the son of the beloved the firstborn before the son of the hated, who is the firstborn: ¹⁷but he shall acknowledge the firstborn, the son of the hated, by giving him a double portion of all that he has; for he is the beginning of his strength; the right of the firstborn is his.