… is from page 176 of volume III (“The Political Order of a Free People,” 1979) of Hayek’s Law, Legislation, and Liberty  (original emphasis):
Man is not and never will be the master of his fate; his very reason always progresses by leading him into the unknown and unforeseen where he learns new things.
True progress cannot be planned. True progress is necessarily open-ended. True progress, therefore, requires that people be free not only to launch new excursions into the unknown, but also to adjust incrementally as time passes to the unfolding unforeseen consequences, along multiple margins, and in ways that are judged best by the actual people on the spot. It is these people who can best assess the new opportunities, constraints, and knowledge that arise because of the changes. True progress by its very nature cannot guarantee that no one’s expectations and hopes and position in society will ever be dashed and threatened.