… is from page 347 of Karl Popper’s 1963 collection, Conjectures and Refutations: The Growth of Scientific Knowledge; specifically, it’s from Popper’s 1956 essay (originally a 1954 address to the Mont Pelerin Society) entitled “Public Opinion and Liberal Principles”:
There is, first, the classical myth, vox populi vox dei, which attributes to the voice of the people a kind of authority and unlimited wisdom. Its modern equivalent is faith in the ultimate common-sense rightness of that mythical figure, ‘the man in the street’, his vote, and his voice. The avoidance of the plural in both cases is characteristic. Yet people are, thank God, seldom univocal; and the various men in the various streets are as different as any collection of V.I.P.s in a conference-room. And if, on occasion, they do speak more or less in unison, what they say is not necessarily wise. They might be right, or they might be wrong.