Romans 9:9:

For this was how the promise was stated: “At the appointed time I will return, and Sarah will have a son.” (NIV)

For this is the word of promise: “AT THIS TIME I WILL COME, AND SARAH SHALL HAVE A SON.” (NASB)

For this is the word of promise, At this time will I come, and Sarah shall have a son. (KJV)

In Rom 9:8, Paul concludes that “It is not the natural children who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring, i.e., God’s children,” as he proves that God’s word does not fail when a Jew can be found that will not be a co-heir with Christ of the promised glory (Rom 9:6–8).

Paul proves that Isaac was a “child of promise” by reminding his audience of the promise as it can be found in Scripture: “At the appointed time I will return, and Sarah will have a son.” (Gen. 18:10,14)