Hebrews 11:18

of whom it had been said-- 'It is through Isaac that there shall be descendants to bear thy name.'

Romans 9:7-8

Nor, because they are Abraham's descendants, are they all his Children; but-'It is Isaac's children who will be called thy 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, because they are Abraham's descendants, are they all his Children; but-'It is Isaac's children who will be called thy descendants.'
Hebrews 11:18
of whom it had been said-- 'It is through Isaac that there shall be descendants to bear thy name.'