Hebrews 11:18

he to whom it was said, "From Isaac shall thine offspring be reckoned;"

Romans 9:7-8

nor because they are descendants of Abraham are they all children; but, "Thy offspring shall be reckoned from Isaac."

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 descendants of Abraham are they all children; but, "Thy offspring shall be reckoned from Isaac."
Hebrews 11:18
he to whom it was said, "From Isaac shall thine offspring be reckoned;"