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.

Genesis 21:1-7   1. And the LORD visited Sarah as he had said, and the LORD did unto Sarah as he had spoken. 2. For Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him. 3. And Abraham called the name of his son that was born unto him, whom Sarah bare to him, Isaac. 4. And Abraham circumcised his son Isaac being eight days old, as God had commanded him. 5. And Abraham was an hundred years old, when his son Isaac was born unto him. 6. ¶And Sarah said, God hath made me to laugh, so that all that hear will laugh with me. 7. And she said, Who would have said unto Abraham, that Sarah should have given children suck? for I have born him a son in his old age.