I did not notice this the first time I was watching the movie but they were actually singing parts of the witches chants from Macbeth in Harry Potter and the Prisoner of Azkaban. I found it while on YouTube looking for something funny from Reduced Shakespeare Company (RCS) to post.
Here is the original text from Act 4 scene 1 of Macbeth.
Double, double toil and trouble;
Fire burn, and cauldron bubble.
Fillet of a fenny snake,
In the cauldron boil and bake;
Eye of newt and toe of frog,
Wool of bat and tongue of dog,
Adder’s fork and blind-worm’s sting,
Lizard’s leg and howlet’s wing,
For a charm of powerful trouble,
Like a hell-broth boil and bubble.
Is there any others that you guys also liked?