Comprehensive Rules

  The source of damage is the object that dealt it. If an effect requires a player to choose a source of damage, they may choose a permanent; a spell on the stack (including a permanent spell); any object referred to by an object on the stack, by a prevention or replacement effect that’s waiting to apply, or by a delayed triggered ability that’s waiting to trigger (even if that object is no longer in the zone it used to be in); or a face-up object in the command zone. A source doesn’t need to be capable of dealing damage to be a legal choice. See rule 609.7, “Sources of Damage.”