Deuteronomy 21:15-17 - The Right Of The Firstborn
15 When a man has two wives, one beloved and the other hated, and they have born him sons, both the beloved and the hated; and if the firstborn son is of the one that was hated, 16 then it shall be, when he makes his sons to inherit that which he has, that he may not give the right of the firstborn unto the son of the beloved in preference over the son of the hated, who is indeed the firstborn; 17 but he shall acknowledge the son of the hated as the firstborn 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.