… is from page 192 of Hayek’s 1978 collection, New Studies in Philosophy, Politics, Economics, and the History of Ideas ; specifically, it’s from a 1972 essay entitled “The Campaign Against Keynesian Inflation”:
What we are experiencing are simply the economic consequences of Lord Keynes. It was on the advice and even urging of his pupils that governments everywhere have financed increasing parts of their expenditure by creating money on a scale which every reputable economist before Keynes would have predicted would cause precisely the sort of inflation we have got. They did this in the erroneous belief that this was both a necessary and a lastingly effective method of securing full employment.
The seductive doctrince that a government deficit, as long as unemployment existed, was not only innocuous but even meritorious was of course most welcome by politicians.