What are the origins of the phrase "by night and fog"?

I know that the phrase "bei nacht und nebel" (by night and fog/by cloak and dagger) referred to people disappearing into the night and deported to concentration camps, never to be seen from again. But what are the origins of this quote? I think it may be Heinrich Himmler, but if someone can tell me which executive order # this is or the quote in its entirety, it'd be appreciated!