Genesis 17:15-19

And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be. And I will bless her, and moreover I will give thee a son of her: yea, I will bless her, and she shall be a mother of nations; kings of peoples shall be of her. Then Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is a hundred years old? and shall Sarah, that is ninety years old, bear? And Abraham said unto God, Oh that Ishmael might live before thee! And God said, Nay, but Sarah thy wife shall bear thee a son; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant for his seed after him.