Genesis 17:19-21

17:19 God said, “No, Sarah your wife is going to bear you a son, and you will name him Isaac. I will confirm my covenant with him as a perpetual covenant for his descendants after him. 17:20 As for Ishmael, I have heard you. I will indeed bless him, make him fruitful, and give him a multitude of descendants. He will become the father of twelve princes; I will make him into a great nation. 17:21 But I will establish my covenant with Isaac, whom Sarah will bear to you at this set time next year.”

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.