Hebrews 11:18

of whom it had been said, It is through Isaac that your posterity will be traced.

Romans 9:7-8

they are not all children of Abraham because they are Abraham's descendants. The promise was, 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
they are not all children of Abraham because they are Abraham's descendants. The promise was, In Isaac shall thy posterity be called.
Hebrews 11:18
of whom it had been said, It is through Isaac that your posterity will be traced.