Deuteronomy 21:17

Instead, he must acknowledge the firstborn, the son of his unloved wife, by giving him a double portion of all that he has. For that son is the firstfruits of his father’s strength; the right of the firstborn belongs to him.

But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his.