Player shows one card during the hand. What now?

[crookdimwit]

Tonight at my home game (NL hold'em, cash game) we had a dispute when a player got heads up against an opponent on the river. The first player went all-in. The second player thought for a bit, then showed one of his cards to the first player (showing that he had trips), trying to get a reaction, but didn't declare any action.

A couple players argued that by showing his card, his hand was dead. Others disagreed, including me. This set off an argument about the rules for handling this situation. As the host, I ruled that the hand was still live, since we had never established any such rule at our game, and with a big pot at stake, I wasn't going to declare the second player's hand dead.

For the future, however I want to establish a house rule on this and make sure everyone understands the rules. After the game, I tried to find a rule on this in Roberts Rules of Poker or elsewhere and can't seem to find anything definitive.

Am I missing a clearly established rule on this anywhere? How is this handled elsewhere?