Genesis 21:1-5

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. Abraham gave the name Isaac to the son Sarah bore him. When his son Isaac was eight days old, Abraham circumcised him, as God commanded him. Abraham was a hundred years old when his son Isaac was born to him.