For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. (NIV)
For it is written that Abraham had two sons, one by the bondmaid and one by the free woman. [Gen. 16:15; 21:2, 9.] (AMP)
For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman. (KJV)