Genesis 17
15 Ref And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be.
16 Ref 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.
17 Ref 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?
18 Ref And Abraham said unto God, Oh that Ishmael might live before thee!
19 Ref 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.