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, Russell Jackson, Joseph Bristow, Ian Small (2000). “The Complete Works of Oscar Wilde: The picture of Dorian Gray : the 1890 and 1891 texts”, p.25, Oxford University Press on Demand