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Roberts Rules of Poker. Section 3 - The Showdown
5. Any player who has been dealt in may request to see any hand that has been called, even if the opponent's hand or the winning hand has been mucked. However, this is a privilege that may be revoked if abused. If a player other than the pot winner asks to see a hand that has been folded, that hand is dead. If the winning player asks to see a losing player’s hand, both hands are live, and the best hand wins.