Epictetus (1758). “All the Works of Epictetus: Which are Now Extant; Consisting of His Discourses, Preserved by Arrian, in Four Books, the Enchiridion, and Fragments”, p.179
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."