Hebrews 11:18

of whom it had been said, "Through Isaac your offspring must be traced."

Romans 9:7-8

nor are they all children of Abraham, because they are his descendants, but the promise was "In the line of Isaac your descendants will I be counted."

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 his descendants, but the promise was "In the line of Isaac your descendants will I be counted."
Hebrews 11:18
of whom it had been said, "Through Isaac your offspring must be traced."