From Pathfinder Howl of the Wild, which isn't enabled.

Force of Nature

FEAT 16

You're a nigh-unstoppable killing machine, with only silver able to quell your rampage. When in hybrid or animal shape, you gain fast healing 5. If you take damage from a silver weapon, your fast healing deactivates until the end of your next turn.

Effect: Force of Nature (Silver Damage)

Pathfinder Howl of the WildORC