Genesis 17:15-19   15. ¶And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be. 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. 17. Then Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear? 18. And Abraham said unto God, O that Ishmael might live before thee! 19. And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him.

Genesis 18:10-14   10. And he said, I will certainly return unto thee according to the time of life; and, lo, Sarah thy wife shall have a son. And Sarah heard it in the tent door, which was behind him. 11. Now Abraham and Sarah were old and well stricken in age; and it ceased to be with Sarah after the manner of women. 12. Therefore Sarah laughed within herself, saying, After I am waxed old shall I have pleasure, my lord being old also? 13. And the LORD said unto Abraham, Wherefore did Sarah laugh, saying, Shall I of a surety bear a child, which am old? 14. Is any thing too hard for the LORD? At the time appointed I will return unto thee, according to the time of life, and Sarah shall have a son.

Romans 9:6-8   6. Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel: 7. Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called. 8. That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.