It appears to be the pulling together, and editing of two different sentences in an exchange between the Dean of the architectural school as they expelled Roark. Referring to designing modern architecture, the dean is saying, "My dear fellow, who will let you?” And Roark replies, "That’s not the point. The point is, who will stop me?” Then the two statements were put together. The concept is clearly Rands, but the words were changed about a bit... while keeping the concept - for the most part.
Here is where I got that, and they also mention the Time's Article, the t-shirt, and others who have used this "quote" that isn't quite a quote:
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I just realized that Stephen Boydstun, in post #5, used the exact same source - the QuoteInvestigator. Sorry, Stephen, I wasn't trying to steal your effort.
(Edited by Steve Wolfer on 11/27, 10:41am)