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 (1997). “Collected Works of Oscar Wilde: The Plays, the Poems, the Stories and the Essays Including De Profundis”, p.701, Wordsworth Editions