Deuteronomy 21:16-17

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 before the son of the hated, who is the first-born: 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.