… is from page 173 of Arnold Plant’s 1970 essay “Homage to Hayek” as this essay is reprinted (and retitled) in the 1974 collection of some of Plant’s writings, Selected Economic Essays and Addresses:
Those whose advocacy of social change is powered by strong emotion have often been repelled by the cold-blooded nature of his [Hayek’s] purely intellectual approach. They would do well to reflect that they would not wish a surgeon to operate on them while his hands were trembling with emotion.