Historical Rules · 1 February 2012
Rule 707.9
Official source text from the 1 February 2012 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. At the end of each game, all face-down permanents and spells must be revealed to all players.