Historical Rules · 1 October 2009
Rule 710.4a
Official source text from the 1 October 2009 Rules release
The controller of another player's turn can use only that player's resources (cards, mana, and so on) to pay costs for that player. Example: If the controller of the turn decides that the active player will cast a spell with an additional cost of discarding cards, the cards are discarded from the active player's hand.