Mucking and showing hands

[CraigJ]

Player 1 bets on the river. Player 2 calls. Player 2 shows his hand first and player 1 mucks his hand declaring player 2 the winner. Player 2 says he wants to see player 1's cards. Player 2 argues he paid to see the cards. Player 1 argues as long as he declares player 2 the winner he doesn't have to show his cards. Who is correct? Is there any set rule for this or does it depend on individual table rules?