A werewolf can be killed only with a silver bullet, or a silver knife or a stick with a silver handle.
8 seconds sound clip from the The Wolf Man (1941) movie soundboard.
You can hear this line at 00:36:20.760 in the Blu-ray version of the movie.
Quote context
[...]
- I remember you. That night, and in the crypt.
- Well, there's no crime in that, is there?
- You think I don't know the difference between a wolf and a man?
- Bela became a wolf, and you killed him.
- I tell you, I killed a wolf! A plain, ordinary wolf!
- Take this charm. The pentagram, the sign of the wolf. It can break the evil spell.
- Evil spell, pentagram, wolfsbane. I'm sick of the whole thing! I'm gonna get out of here.
[...]