This is found poem taken from the following section of Harry Potter and the Prisoner of Azkaban:

"Though Harry had by no means forgotten about Black, he couldn’t brood constantly on revenge if he wanted to help Hagrid win his case against the Committee for the Disposal of Dangerous Creatures. He, Ron, and Hermione went to the library the next day and returned to the empty common room laden with books that might help prepare a defense for Buckbeak. The three of them sat in front of the roaring fire, slowly turning the pages of dusty volumes about famous cases of marauding beasts, speaking occasionally when they ran across something relevant."