NL ring game - must show hands when all in?

[doubledown998]

I was playing in a 1/2 NL ring game last week, and myself and another player eventually were All In pre flop.

The dealer would not lay down the flop until we both turned our cards over.

I thought that this only applied to tournaments, that you must show your hole cards?

If this is indeed a hard and fast rule, anyone have a link to a rules site showing said rule?