Lazy Nazgul

[Gordis]

You must have referred to this quote then:

[The nazgul]came to Sarn Ford and the southernmost borders of the Shire. They found them guarded for the Rangers barred their way. But this was a task beyond the power of the Dunedain; and maybe it would still have proved a even if their captain, Aragorn, had been with them. But he was away to the north, upon the East Road near Bree; and the hearts even of the Dunedain misgave them.

Nothing is said that they sent for Aragorn, instead it is told that his presence likely wouldn't have made much difference.