Attributed in Reader's Digest, Sept. 1937. The attribution to Twain is obviously spurious because Twain's father died when the future writer was eleven years old.
Wallace D. Wattles (2015). “The Science of Wallace D. Wattles: The Science of Being Well, The Science of Getting Rich & The Science of Being Great – Complete Trilogy: From one of the New Thought pioneers, author of How to Promote Yourself, New Science of Living and Healing, Hellfire Harrison, A New Christ, How to Get What You Want and Jesus The Man and His Work”, p.169, e-artnow