Historical Rules · 18 February 2022
Rule 718.5a
Official source text from the 18 February 2022 Rules release
The controller of another player can use only that player’s resources (cards, mana, and so on) to pay costs for that player. Example: If the controller of a player decides that the controlled player will cast a spell with an additional cost of discarding cards, the cards are discarded from the controlled player’s hand.