Attributed to Ludwig van Beethoven by Bettina von Arnim in a letter to Goethe (May 28, 1810), as quoted in Bettina von Arnim "Goethe's Correspondence with a Child", 1835.
Ludwig van Beethoven, Friedrich Kerst, Henry Edward Krehbiel (1964). “Beethoven, the Man and the Artist, as Revealed in His Own Words”, p.13, Courier Corporation
Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.12, St. Martin's Press