The Importance of Being Earnest act 1 (1895). The same lines appear, as a dialogue between Lord Illingworth and Mrs. Allonby, in A Woman of No Importance, act 2 (1893).
Mark Twain (2014). “Mark Twain on Common Sense: Timeless Advice and Words of Wisdom from America's Most-Revered Humorist”, p.13, Skyhorse Publishing, Inc.
Oscar Wilde, Russell Jackson, Ian Small (2000). “The Complete Works of Oscar Wilde: De profundis, "Epistola : in carcere et vinculis"”, p.95, Oxford University Press on Demand