Genesis 17:19

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.

Genesis 17:21

17:21 But I will establish my covenant with Isaac, whom Sarah will bear to you at this set time next year.”

Romans 9:7-8

9:7 nor are all the children Abraham’s true descendants; rather “through Isaac will your descendants be counted.” 9:8 This means it is not the children of the flesh who are the children of God; rather, the children of promise are counted as descendants.

Hebrews 11:18

11:18 God had told him, “Through Isaac descendants will carry on your name,”