Rule 615.4

Prevention effects must exist before the appropriate damage event occurs—they can’t “go back in time” and change something that’s already happened. Spells or abilities that generate these effects are often cast or activated in response to whatever would produce the event and thus resolve before that event would occur. Example: A player can activate an ability that prevents damage in response to a spell that would deal damage. Once the spell resolves, though, it’s too late to prevent the damage.