Romans 9:6-10

Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel:

Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.

That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.

For this is the word of promise, At this time will I come, and Sara shall have a son.

And not only this; but when Rebecca also had conceived by one, even by our father Isaac;