Stun

• WWizards of the Coast2004-10-04
  Official Ruling
  Must be used before blockers are declared in order to affect blocking decisions. You can’t wait to see what your opponent declares and then try to stop them.
• WWizards of the Coast2004-10-04
  Official Ruling
  You can use it after combat or even on a creature which can’t possibly block this turn because it’s that player’s turn to attack, but it generally has no effect other than to let you draw a card.

Rulings via Scryfall