Where’s that from?
The Big Cheese?
Goudafellas
The Swiss Connection?
Well, according to the top result on Google, it’s from this very thread. And you said it.
Hope that helps!
I’ve never heard this quote before, and a quick Google on “I have always been Swiss” only produces 10 hits. One is this thread. Several use the phrase in a different context. The only relevant matches seem to be a few posts on different recipe forums, quite possibly all by the same person.
I think it’s safe to say this is not a famous quote, and unless you heard it elsewhere then it seems likely to me that the recipe forum poster either invented or misremembered it.
It’s from Switzerland.
I am swish. I have always been swish, and will always be swish.
As a Swiss I’ve never heard it.
But I am Swiss, I have always been Swiss and will always be Swiss. 
Well, perhaps they are thinking of President Obama?
According to this news blurb (the article is in Swiss German, not in English) genealogists traced Obama’s ancestry back 300 years to his great, great, great, great grandparents and found them to be Swiss who then moved to the USA. Thus, Obama could make a case that he was Swiss and could buy some lederhosen and a chalet and retire near the Matterhorn someday, should he so choose.
Don’t recall that I’ve ever heard that, but in an episode of The Kallikaks, a telephone company rep named Hindeman (intended to be evocative of both a disgraced Nixon staffer and a fugitive ex-Nazi) protests that he is not German, but Swiss. Might be the source of your quote, as it echoes popular perception of Nixon’s “I am not a crook” sound bite.
(In an early exchange, the Kallikaks, who have received an excessive refund from the phone company, are accused by Hindeman. Jasper Kallikak protests that he is not a crook, and Hindeman smirks knowingly and says “That’s what we all say.” Later, when Oscar Heinz, Jasper’s German employee shows up, he greets Hindeman delightedly as “Countryman!” Hindeman replies testily, “I told you, I’m Swiss!” Oscar winks broadly, nods, and says “Ja, that’s what we all say!” I think Hindeman might have told them he was Swiss with the words in the OP, or something like them.)
I can’t remember when or where I heard it. Possibly in the late-'70s from a classmate who saw it somewhere, or possibly on a sketch-comedy show, other television show, or movie in the '80s. I’m pretty sure I saw it. The context was that someone was insinuated to be a former-Nazi. He says the line in a German accent.
I believe it’s the official motto of ODESSA.
More seriously, it’s from The Secret War of Harry Frigg (1968).
It’s a now very old joke from the immediate post-Second World War era, allegedly from B movies.
F’rinstance, a loud shout by a man on the street: “Hey, [stereotypical German first name]!” directed to someone walking down the street and whose back is toward the person shouting.
Person with stereotypical German first name stops dead in his tracks, quickly raises his hands in the air and shouts in heavily accented German, “I am Swiss!”
It’s in the same league as the stereotypical British PoW asking a suspected enemy infiltrator spy keeping an eye on the escape tunnel being dug, “What’s the time?” in German. The suspected infiltrator always answers in German.
Man, everyone wants to be Swiss.
Obama, Gymnopithys, the Kallikaks, my Dad’s side of the family…
even Movie Nazis.
In the book/movie the Exorcist, the character Karl has a similar line. It’s not that lengthy – he just says, “I am Swiss” when Burke Dennings refers to him as German/a Nazi.
Mein Gott im Himmel, you not only watched that show (solid contender for The Worst Sitcom of All Time) but also remember specifics? What a horrible burden you’ve borne for four decades!
It kind of sounds like something from Fawlty Towers or Tom Lehrer’s I Was Not A Nazi Polka
I remember that scene. It’s possible that a classmate, years later, used the quote as a basis for a longer, more amusing line.
When you put it that way, it sounds kind of familiar. Like something out of Monty Python or The Producers.