18:1 And the Lord appeared to him by the oaks of Mamre, as he sat at the door of his tent in the heat of the day. 2 He lifted up his eyes and looked, and behold, three men were standing in front of him. When he saw them, he ran from the tent door to meet them and bowed himself to the earth 3 and said, “O Lord, if I have found favor in your sight, do not pass by your servant. 4 Let a little water be brought, and wash your feet, and rest yourselves under the tree, 5 while I bring a morsel of bread, that you may refresh yourselves, and after that you may pass on—since you have come to your servant.” So they said, “Do as you have said.” 6 And Abraham went quickly into the tent to Sarah and said, “Quick! Three seahs of fine flour! Knead it, and make cakes.” 7 And Abraham ran to the herd and took a calf, tender and good, and gave it to a young man, who prepared it quickly. 8 Then he took curds and milk and the calf that he had prepared, and set it before them. And he stood by them under the tree while they ate.
9 They said to him, “Where is Sarah your wife?” And he said, “She is in the tent.” 10 The Lord said, “I will surely return to you about this time next year, and Sarah your wife shall have a son.” And Sarah was listening at the tent door behind him. 11 Now Abraham and Sarah were old, advanced in years. The way of women had ceased to be with Sarah. 12 So Sarah laughed to herself, saying, “After I am worn out, and my lord is old, shall I have pleasure?” 13 The Lord said to Abraham, “Why did Sarah laugh and say, ‘Shall I indeed bear a child, now that I am old?’ 14 Is anything too hard for the Lord? At the appointed time I will return to you, about this time next year, and Sarah shall have a son.” 15 But Sarah denied it, saying, “I did not laugh,” for she was afraid. He said, “No, but you did laugh.”
21:1 The Lord visited Sarah as he had said, and the Lord did to Sarah as he had promised. 2 And Sarah conceived and bore Abraham a son in his old age at the time of which God had spoken to him. 3 Abraham called the name of his son who was born to him, whom Sarah bore him, Isaac. 4 And Abraham circumcised his son Isaac when he was eight days old, as God had commanded him. 5 Abraham was a hundred years old when his son Isaac was born to him. 6 And Sarah said, “God has made laughter for me; everyone who hears will laugh over me.” 7 And she said, “Who would have said to Abraham that Sarah would nurse children? Yet I have borne him a son in his old age.” (ESV)