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, Peter Raby (2008). “The Importance of Being Earnest and Other Plays: Lady Windermere's Fan; Salome; A Woman of No Importance; An Ideal Husband; The Importance of Being Earnest”, p.226, Oxford Paperbacks
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.78, Oxford University Press on Demand