Actually, the quote is correct as is, although the version you mention is all over the Internet as well.
Algernon says the line in response to Jack in Act 1 of The Importance of Being Earnest...
"The truth is rarely pure and never simple"
(Importance of Being Earnest, Act I).
The exchange goes like this:
My dear Algy, I don't know whether you will be able to understand my real motives. You are hardly serious enough. When one is placed in the position of guardian, one has to adopt a very high moral tone on all subjects. It's one's duty to do so. And as a high moral tone can hardly be said to conduce very much to either one's health or one's happiness, in order to get up to town I have always pretended to have a younger brother of the name of Ernest, who lives in the Albany, and gets into the most dreadful scrapes. That
, my dear Algy, is the whole truth pure and simple
ALGERNON: The truth is rarely pure and never simple.
Modern life would be very tedious if it were either, and modern literature a complete impossibility!
~from The Importance of Being Earnest, Act 1, by Oscar Wilde
Check out this link to a description of a MUST SEE movie about Wilde called "Wilde"