… is from page 218 of the 2014 collection, The Market and Other Orders  (Bruce Caldwell, ed.), of some of F.A. Hayek’s essays on spontaneous-ordering forces; specifically, this quotation is from Hayek’s June 1962 lecture, “The Economy, Science and Politics” (available on-line here ):
The power of any particular union to push up the wages of its members, that is, to make them higher than they would be without the activity of the union, rests entirely on its ability to prevent the entry into the trade of workers willing to work for a lower wage. This will have the effect that the latter either must work elsewhere at still lower wages or that they will remain unemployed.