Romans 9:8-12

That is, it is not the children of the flesh that are children of God; but the children of the promise are reckoned for a seed. For this is a word of promise, According to this season will I come, and Sarah shall have a son. And not only so; but Rebecca also having conceived by one, even by our father Isaac— for the children being not yet born, neither having done anything good or bad, that the purpose of God according to election might stand, not of works, but of him that calleth, it was said unto her, The elder shall serve the younger.