Stop That is a common Instant card from Unhinged (UNH), collector number 63. Current market price: $0.20 (foil $12.94), TCGplayer market prices via Scryfall.
Target player discards a card. Gotcha — If an opponent audibly flicks the cards in their hand, you may say "Gotcha!" When you do, return this card from your graveyard to your hand.