Romans 9:8-9 — 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 Sarah shall have a son.
The quote is from Genesis 18:10 — And he said, I will certainly return unto thee according to the time of life; and, lo, Sarah thy wife shall have a son. And Sarah heard it in the tent door, which was behind him.
Abraham and Sarah planned according to the flesh and tried to help fulfill God's promise. Ishmael was the result, but God is sovereign in His selection and choose Isaac.
They who are the children of the flesh — only of the flesh and not also of the promise because the children of promise are also of the flesh.
Promise — not just that Isaac would come, but that he would come through God's power.