"Students of social science, must fear popular approval: Evil is with them when all men speak well of them. If there is any set of opinions by the advocacy of which a newspaper can increase its sales, then the student who wishes to leave the world in general and his country in particular better than it would have been if he had not been born, is bound to dwell on the limitations and defects and errors, if any, in that set of opinions: and never to advocate them unconditionally even in ad hoc discussion. It is almost impossible for a student to be a true patriot and to have the reputation of being one in his own time."
Hayek quoting from an comment from Marshall which appears in "In Memoriam: Alfred Marshall," a speech given by A.C. Pigou in 1924 and published as part of a Memorials of Alfred Marshall volume in 1925 (pp. 81-90). The fuller quotation attributed to Marshall (on p. 89).