Addendum: A helpful reader points out that a similar comment was attributed to Ronald Coase by David Landes, who said in a 1983 symposium published in the Journal of Law and Economics: "Ronald said he had gotten tired of antitrust because when the prices went up the judges said it was monopoly, when the prices went down, they said it was predatory pricing, and when they stayed the same, they said it was tacit collusion."
The citation is Edmund W. Kitch, "The Fire of Truth: A Remembrance of Law and Economics at Chicago, 1932-1970," Journal of Law & Economics, Vol. 26, No. 1 (Apr., 1983), pp. 163-234. Quotation is on p. 193.