Hebrews 11:18

with regard to whom he had been told, "It is through Isaac that your posterity shall be traced."

Romans 9:7-8

nor because they are Abraham's true children. But the promise was "Through Isaac shall your posterity be reckoned."

Treasury of Scripture Knowledge

Summary

And God said unto Abraham, Let it not be grievous in thy sight because of the lad, and because of thy bondwoman; in all that Sarah hath said unto thee, hearken unto her voice; for in Isaac shall thy seed be called.

Bible References

In isaac

Romans 9:7
nor because they are Abraham's true children. But the promise was "Through Isaac shall your posterity be reckoned."
Hebrews 11:18
with regard to whom he had been told, "It is through Isaac that your posterity shall be traced."