Dealer question -- first to show wins pot?

[fraserbrown]

Had a disagreement about this very type of thing last night. I hit the nut flush on the turn and put my only caller all-in, he calls and the river comes a blank. I flip my cards quickly and opponent goes to muck, but before they hit the pile I ask to see. Opponent says he doesnt have to show if he doesnt want to, myself and another player tell him he has to if he called. Dealer refuses to turn over the cards and I say I paid and want to see what he called with, dealer tells me I won the pot and dont need the info, so the guy next to me reaches over and flips the cards which show top pair with no kicker. Row ensues and we both get a warning.