Benjamin Franklin's letter to David Hartley (December 4, 1789) as quoted in "The Writings of Benjamin Franklin," Volume 10 (p. 72) edited by Albert H. Smyth, 1907.
Benjamin Franklin (1987). “Poor Richard's Almanack: Being the Choicest Morsels of Wisdom, Written During the Years of the Almanack's Publication”, p.22, Peter Pauper Press, Inc.