Deuteronomy 21:15-17

Young's Literal Translation

¹⁵ `When a man hath two wives, the one loved and the other hated, and they have borne to him sons (the loved one and the hated one), and the first-born son hath been to the hated one;

¹⁶ then it hath been, in the day of his causing his sons to inherit that which he hath, he is not able to declare first-born the son of the loved one, in the face of the son of the hated one -- the first-born.

¹⁷ But the first-born, son of the hated one, he doth acknowledge, to give to him a double portion of all that is found with him, for he [is] the beginning of his strength; to him [is] the right of the first-born.