Canada Month, June 1963. A later satirical version of this saying added to the end "Luckily, this is not difficult" (Paul Dickson, The Official Rules [1978]). See Eleanor Roosevelt 2
Oscar Wilde (1997). “Collected Works of Oscar Wilde: The Plays, the Poems, the Stories and the Essays Including De Profundis”, p.124, Wordsworth Editions
Rudyard Kipling (2016). “Collected Works of Rudyard Kipling (Illustrated Edition): 5 Novels & 350+ Short Stories, Poetry, Historical Military Works and Autobiographical Writings from one of the most popular writers in England, known for The Jungle Book, Kim, The Man Who Would Be King”, p.724, e-artnow (Open Publishing)
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).