Historical Rules · 1 June 2014
Rule 712.5a
Official source text from the 1 June 2014 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.