Deuteronomy 21:16-17

When the man divides his inheritance, he may not give the larger inheritance to his younger son, the son of the wife he loves, as if he were the firstborn son. He must recognize the rights of his oldest son, the son of the wife he does not love, by giving him a double portion. He is the first son of his father’s virility, and the rights of the firstborn belong to him.