That's the one.
1 seconds sound clip from Scooby-Doo, Where Are You! - The Backstage Rag (S1E9).
You can hear this line at 00:15:58.758 in the Blu-ray version of the episode.
Quote context
[...]
- Somebody's playing the big organ on the stage.
- It must have been our imagination.
- Well, that was the first time I ever heard mine.
- I don't like looking for clues anymore.
- Look! Scooby has a dog puppet.
- So this is the canine Mata Hari.
- What's sticking out of that puppet's stuffing?
- What a clue! It's the front engraving plate for a $20 bill!
- It's time we wake that doorman up and tell him what's going on.
[...]