Back to Methuselah pt. 1, act 1 (1921). This was a favorite quotation of Robert F. Kennedy's, and Edward M. Kennedy used it in his eulogy of Robert Kennedy.
Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.32, St. Martin's Press
H. Jackson Brown Jr. (2007). “Complete Life's Little Instruction Book: 1,560 Suggestions, Observations, and Reminders on How to Live a Happy and Rewarding Life”, p.370, Thomas Nelson Inc