I haven't really found where Hayek got this quote from. Someone linked an Italian speech but I didn't quite get an English translation that had it.
Yanked my copy of The Road to Serfdom off the shelf, and check for Mussolini in the index.
Hayek provides the quote as the epitaph to chapter four, "The 'Inevitability' of Planning." There is an editor's note at the bottom of the page which reads:
[Benito Mussolini, Grand Fascist Council Report, 1929, quotes in E.B. Ashton, The Fascist: His State and His Mind (New York: William Morrow and Co., 1937), p. 63, note 5 – Ed.]
So I guess I would try to find the Ashton book.