Deuteronomy 21:17

but he shall acknowledge the son of the hated for the first-born, by giving him a double portion of all that he hath, of all the property found with him, for thus the firstborn was distinguished among the children of Israel; for he is the beginning of his strength, as Reuben was that of Jacob, Genesis 49:3; the right of the first-born is his. The Lord wants no favoritism shown at any time, but demands that right and justice rule among His people.