The Wolf Man
The Wolf Man, 1941 film. Its most famous quote is made by the gypsy Maleva (Maria Ouspenskaya) to Larry Talbot (Lon Chaney Jr.):
“Even a man who is pure of heart, and says his prayers by night, may become a wolf when the wolfbane blooms, and the autumn moon is bright.”
Various sources differ on the exact wording, probably because this quote is repeated by other characters. Maleva's version would clearly be authoritative, as a she is a gypsy from the Old Country, and her son Bela (Bela Lugosi) is a werewolf.
Corrections to the quote (the Wolf Man Curse) are welcome from someone who actually watches the film, or at least listens to the soundtrack.