Quoted in Bennett Cerf, Try and Stop Me: A Collection of Anecdotes and Stories, Mostly Humorous (1944). Although this is usually credited to Mizner, the Los Angeles Times, 17 Mar. 1941, quotes Bob Oliver: "If you steal from one man, it's plagiarism. If you steal from several, it's research."
"Laconic Manual and Brief Remarker, containing over a thousand subjects alphabetically and systematically arranged". Book by Charles Simmons, p. 20. Chapter: "Accuracy", 1852.