Cheapest priced copy (Revised Edition): $0.35 at Card Kingdom, near mint, their price list as of .
Circle of Protection: Red 
Enchantment · Common · Ice Age · #15
{1}: The next time a red source of your choice would deal damage to you this turn, prevent that damage.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
33 printings · $0.35 to $49.99 at Card Kingdom
2 rulings from Wizards of the Coast
- 4 October 2004Can be used even when there is no damage to prevent. It prevents the next damage (if any) from the source this turn.
- 4 October 2004A 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.