“No, it's your wife Sarah who is going to have a son for you!” God replied. “You are to call him Isaac. I will keep my agreement with him and his descendants as an eternal agreement.
Then one said, “I promise you that I will come back to visit you about this time next year, and your wife Sarah will have a son.” Sarah was listening, hiding just inside the entrance to the tent behind him.
God also gave Abraham the agreement regarding circumcision, and so when Isaac was born, Abraham circumcised him on the eighth day. Isaac was the father of Jacob, and Jacob the father of the twelve patriarchs.
By her trust in God even Sarah herself was given the ability to conceive a child though she was too old, because she trusted God who had made the promise.