Showdown Scenario where bluffer does not want to show

[all_in_lam]

scenario: assuming heads up.. there is a bet/raise on river.

i udnerstand that the last bet/raise has to show first, but what happens if the person says he has nothing, i was bluffing. does the winner have to show his hand?

i know that if bettor/raiser mucks.. the winner does not have to show unless somoene asks, but what if the person jst says "nothign, u win".

is it ethical to show the winning hand, or obligatory?