Historical Rules · 26 August 2016
Rule 707.9
Official source text from the 26 August 2016 Rules release
If a face-down permanent moves from the battlefield to any other zone, its owner must reveal it to all players as he or she moves it. If a face-down spell moves from the stack to any zone other than the battlefield, its owner must reveal it to all players as he or she moves it. If a player leaves the game, all face-down permanents and spells owned by that player must be revealed to all players. At the end of each game, all face-down permanents and spells must be revealed to all players.