All-in Rules Dispute

[Vanquish]

Okay, this is dumb, yes, but I have a player in a home-game who doesn't like to show his cards when he's all-in unless he wins the pot. Even if he initiated the action by going all-in, he'll get called, end up losing and just plain old not show his cards.

I know there is a rule that says when you are all-in and there is no further side action that you have to flip your cards. I can't seem to find a refernce in anything "official" to back this up. I checked Robert's Rules and other stuff at HomePokerTourney.com but I can't seem to find it. When I call him out on it I'd like to have something official to point to beyond the rule that if you were the last to show aggression you have to show your hand first.

Thanks!