Deuteronomy 21:15-17 - The Right Of The Firstborn
15 If a man has two wives, the one beloved, and the other regarded inferior, and they have borne him sons, both the beloved and the one regarded inferior, and if the first-born son be hers who was regarded inferior, 16 then it shall be, in the day that he causes his sons to inherit that which he has, that he may not make the son of the beloved the first-born before the son of the one regarded inferior, who is the first-born. 17 But he shall acknowledge the first-born, the son of the one regarded inferior, by giving him a double portion of all that he has, for he is the beginning of his strength. The right of the first-born is his.