"Mutabor" is the magic word to transform into an animal (and back again) in which fairy tale, written by Wilhelm Hauff, a German poet and novelist?

anyone who sniffs this powder and then says the Latin word mutabor will be transformed into the animal he chooses and will be able to understand the speech of animals. There is no danger in doing this since in order to resume human form the sniffer has only to bow three times to the east and then repeat the magic word mutabor.