Genesis 17:16
And I will bless her, and give thee a son also of her: yea, I will bless her, and she shall be a mother of nations; kings of people shall be of her. And I will bless her, and give thee a son also of her: yea, I will bless her, and she shall be a mother of nations; kings of people shall be of her.
The promise was given after the birth of Ishmael, when Abraham had lived nearly twenty-five years in Canaan. The Lord appeared again to establish His everlasting covenant with Abraham and his seed after him, declaring Himself the Almighty God and commanding Abraham to walk before Him and be perfect. At this Abraham fell on his face in awe. As a token of the fulfillment of the covenant that Abraham would be a father of many nations, his name was changed from Abram to Abraham, signifying father of a great multitude, and Sarai's name was changed to Sarah, signifying princess. The promise was that Sarah herself would bear Abraham a son, that she would become a mother of nations, and that kings of people would come from her.
Abraham did not yet understand or grasp that the promise would be fulfilled through Sarah, but fixed his mind on Ishmael as the one through whom the many nations would come. In affection he pleaded and wished that Ishmael might live before God. Because of that misunderstanding, the promise was repeated more definitely and specifically that Sarah would bear Isaac. With Isaac and his seed and descendants after him God would establish His everlasting covenant and continue its line through him. The promise that Sarah would have a son was later repeated still more distinctly by angels on their way to destroy Sodom.