Romans 9:8-10

8In other words, it is not the children by physical descent who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring.

9For this was how the promise was stated: “At the appointed time I will return, and Sarah will have a son.” Gen. 18:10,14 10Not only that, but Rebekah’s children were conceived at the same time by our father Isaac.