Is there an official rule book (or how you call it) online for this kind of things to check out?
There is no one, finally authoritative set of rules for poker. However, most online and live cardrooms either base their house rules on or have house rules that are largely similar to the closest thing we have to an authoritative rulebook: Robert's Rules for Poker. These were written by Robert (Bob) Ciaffone, and are generally considered to be the standard by which all other cardroom house rule books are compared. The current revision of these rules are Version 11, available in Word format here or in an HTML version here.
In general, according to Robert's Rules, in order to have any claim on a pot at the showdown, a player must have 2 live cards tabled. If a player refuses to show both cards and there are other active players, that player's hand is declared dead and is mucked.
In practice, what often happens is one player reveals one card and another player mucks his hand, knowing that he has lost. The dealer will typically push the pot to the first player and the game continues. However, going strictly by the rules anyone at the table is within their rights to demand to see the winning player's other card. A player who insists on not showing both cards is operating well outside both the rules of the game and basic etiquette. A player who demands to see the other card is both well within their rights and it is not considered poor etiquette to do so, especially if that other player has a live hand.
It's also very poor form to slow the game down needlessly. Players often get in to this cat-and-mouse game of one player demanding to see both cards and the other player refusing to do so. Don't be that guy. Table your hand right away. There are 8 other players waiting for another hand while you play your little game.