… is from page 23 of Vol. 12 – Economic Inquiry and Its Logic  (2000) – of The Collected Works of James M. Buchanan; specifically, it’s from Jim’s December 1976 Liberty Fund lecture “General Implications of Subjectivism in Economics” (original emphasis):
The principle that exposure to economics should convey is that of the spontaneous coordination which the market achieves. The central principle of economics is not the economizing process; it is not the maximization of objective functions subject to constraints. Once we become methodologically trapped in the maximization paradigm, economics becomes mathematics or engineering.