Circle of Protection: Green

Enchantment

: The next time a green source of your choice would deal damage to you this turn, prevent that damage.

2004-10-04:
A source of damage is a permanent, a spell on the stack (including one that creates a permanent), or any object referred to by an object on the stack. A source doesn’t need to be capable of dealing damage to be a legal choice.
2004-10-04:
Can be used even when there is no damage to prevent. It prevents the next damage (if any) from the source this turn.