Historical Rules · 25 August 2017
Rule 108.3b
Official source text from the 25 August 2017 Rules release
Some spells and abilities allow a player to take cards he or she owns from outside the game and bring them into the game. (See rule 400.10b.) If a card outside that game is involved in a Magic game, its owner is determined as described in rule 108.3. If a card outside that game is in the sideboard of a Magic game (see rule 100.4), its owner is considered to be the player who started the game with it in his or her sideboard. In all other cases, the owner of a card outside the game is its legal owner.