Source, in full
Though at times attributed to Leary on the internet, no published source of this has been located. It is a misquote of a William S. Burroughs reading entitled M.O.B. from the Giorno Poetry Systems boxed set. M.O.B. was an extension of Burroughs' expression in The Place of Dead Roads (1983), p. 155
Notes from Wikiquote
You are a Shit Spotter. It's satisfying work. … We have observed that most of the trouble in the world has been caused by ten to twenty percent of folks who can't mind their own business, because they have no business of their own to mind, any more than a smallpox virus … The mark of a basic shit is that he has to be right.