Pokémon Season 1 Episode 52

When Ash and his friends see a group of Growlithe training against a Policeman posing as a robber, Ash decides for him and Pikachu to do the same training to become the best they can be. However, when Team Rocket comes to steal them, Officer Jenny, Ash, and the gang's voice gets changed, causing the Growlithe to think that Team Rocket are the Officers as they use their ears to follow command.