Hebrews 11:18

of whom he had been told, "Your posterity is to arise through Isaac!"

Romans 9:7-8

nor are they all children of Abraham because they are descended from him, but he was told, "The line of Isaac will be called your descendants."

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 are they all children of Abraham because they are descended from him, but he was told, "The line of Isaac will be called your descendants."
Hebrews 11:18
of whom he had been told, "Your posterity is to arise through Isaac!"