Inside of a Pokeball?

I'm positive I remember seeing ads a number of years ago for an episode of Pokemon in which they were actually inside a Pokeball. Maybe it was Ash's dream or something, I don't know, I've never seen the episode. Other sources have directed me to the episode Pokeball Peril, but I watched that and it's not it. I've been unable to find the video, can anyone please help me out here?