Romans 9:5-10
English Standard Version
6But it is not as though the word of God has failed. For not all who are descended from Israel belong to Israel,
7and not all are children of Abraham because they are his offspring, but "Through Isaac shall your offspring be named."
8This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as offspring.
9For this is what the promise said: "About this time next year I will return, and Sarah shall have a son."
10And not only so, but also when Rebekah had conceived children by one man, our forefather Isaac,