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.)

Genesis 21:12

21:12 But God said to Abraham, “Do not be upset about the boy or your slave wife. Do all that Sarah is telling you because through Isaac your descendants will be counted.