422.1.

If a player realizes that he or she can't legally take an action after starting to do so, the entire action is reversed and any payments already made are canceled. No abilities trigger and no effects apply as a result of an undone action. If the action was playing a spell, the spell returns to the zone it came from. The player may also reverse any legal mana abilities played while making the illegal play, unless mana from them or from any triggered mana abilities they triggered was spent on another mana ability that wasn't reversed. Players may not reverse actions that moved cards to a library or from a library to any zone other than the stack. Players may not reverse actions that involved a random choice or random zone change.