What's the Rule?

We were walking by Amnon Filippi's table when we heard him call for a floor. From what we could piece together, a player bet, Filippi called, and the original bettor mucked. Jonathan Aguiar, who was at the other end of the table, then asked what the rule was regarding whether or not Filippi had to show a winning hand. The floor was called over and stated that any player can request to see a called hand, but players were not required to show their winning hand on their own if their opponent mucks.

"Are you asking to see a called hand?" Filippi asked Aguiar.

Aguiar responded that he was not and simply wanted clarification on the rule. "The rule here could be do ten jumping jacks, I don't know," he joked.