Gen 18:10-14, "Then the LORD said, 'I will surely return to you about this time next year, and Sarah your wife will have a son.' Now Sarah was listening at the entrance to the tent, which was behind him. Abraham and Sarah were already old and well advanced in years, and Sarah was past the age of childbearing. So Sarah laughed to herself and thought, 'After I am worn out and my master is old, will I now have this pleasure?' Then the LORD said to Abraham, 'Why did Sarah laugh and say, 'Will I really have a child, now that I am old?' Is anything too hard for the LORD? I will return to you at the appointed time next year and Sarah will have a son.' "
Gen 21:1-2, 6-7, "Now the LORD was gracious to Sarah as He had said, and the LORD did for Sarah what He had promised. Sarah became pregnant and bore a son to Abraham in his old age, at the very time God had promised him... Sarah said, 'God has brought me laughter, and everyone who hears about this will laugh with me.' And she added, 'Who would have said to Abraham that Sarah would nurse children? Yet I have borne him a son in his old age.' "