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).
Mark Twain (2014). “Mark Twain on Common Sense: Timeless Advice and Words of Wisdom from America's Most-Revered Humorist”, p.61, Skyhorse Publishing, Inc.
Charlotte Bronte, Emily Bronte, Anne Bronte (2009). “The Bronte Sisters: Three Novels: Jane Eyre; Wuthering Heights; and Agnes Grey (Penguin Classics Deluxe Edition)”, p.224, Penguin
William Cowper, James Thomson (1832). “The Works of Cowper and Thompson: Including Many Letters and Poems Never Before Published in this Country. With a New and Interesting Memoir of the Life of Thomson”, p.136