Historical Rules · 5 October 2018
Rule 715.5a
Official source text from the 5 October 2018 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.