Hebrews 11:18

of whom it was said, "only the descendants of Isaac shall be counted your posterity:"

Romans 9:7-8

neither are they who are the race of Abraham, all children: but 'tis said, "in Isaac shall thy posterity be called."

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
neither are they who are the race of Abraham, all children: but 'tis said, "in Isaac shall thy posterity be called."
Hebrews 11:18
of whom it was said, "only the descendants of Isaac shall be counted your posterity:"