I think they were up on the mountain playing with matches, or defacing public property, or...
8 seconds sound clip from the How the Grinch Stole Christmas (2000) movie soundboard.
You can hear this line at 00:08:40 in the Blu-ray version of the movie.
Quote context
[...]
- I don't need to remind you that this Christmas marks the one thousandth Whobilation.
- Whoville's most important celebration!
- No, no, no, no sir, the boys didn't see any Grinch.
- I think they were up on the mountain playing with matches, or defacing public property, or...
- Oh, well. That's a relief. All right, you heard the man, there's no Grinch problem here.
- Lou, I need it there by tomorrow.
- But, Dad. I just don't understand something. Why won't anyone talk about the Grinch?
- You see, the Grinch is a Who who always... Actually, not a Who, he's more of a...
- A what?
- Exactly, honey. And he's a What who doesn't like Christmas.
[...]