To win any part of a pot if more than one player has a hand, a player must show all of his cards faceup on the table, whether they were used in the final hand played or not. Cards speak for themselves: the actual value of a player's hand prevails in the event a player mis-states the value of his hand. Because exposing a losing hand gives information to an opponent, players may be reluctant to expose their hands until after their opponents have done so and will muck their losing hands without exposing them. Robert's Rules of Poker state that the last player to take aggressive action by a bet or raise is the first to show the hand--unless everyone checks (or is all-in) on the last round of betting, then the first player to the left of the dealer button is the first to show the hand.

do i have to show my cards in poker?
yes