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).
Oscar Wilde (2013). “The Picture of Dorian Gray: The Story of a Fashionable Young Man Who Sells His Soul for Eternal Youth and Beauty (Beloved Books Edition)”, p.37, Lulu Press, Inc