Genesis 21:2-5

21:2 So Sarah became pregnant and bore Abraham a son in his old age at the appointed time that God had told him. 21:3 Abraham named his son – whom Sarah bore to him – Isaac. 21:4 When his son Isaac was eight days old, Abraham circumcised him just as God had commanded him to do. 21:5 (Now Abraham was a hundred years old when his son Isaac was born to him.)