It`s supposedly from the movie Wolfman (which I`ve never heard of) but I don't know what other website to go to that actually has the poem and the poet. There are a few different ways of writing it:
"Even a man who is pure in heart and says his prayers by night may become a wolf when the moon is full and bright" --which is the way I first heard it
"Even the man who is pure in heart and say's his prayers by night, may become a wolf when the Wolfbane blooms and the moon is pure and bright"
"Even a man who is pure in heart and says his prayers by night,
may become a wolf when the wolfbane blooms and the Autumn moon is bright"
There may be other versions but these are the ones I know. Does anyone know who wrote this poem?
Thanks!

It was said by Jenny Williams, a character in the movie, The Wolf Man. Imdb and Wikipedia have information about the movie. I didn't check, but Imdb might mention who wrote the screenplay.

One site says the poem is from the werewolf movies of the '40s. The site quoted Lon Chaney, Jr.