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 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.
Romans 9:7
9:7 nor are all the children Abraham’s true descendants; rather “
through Isaac will your descendants be counted.”