Leo Burnett (1961). “Communications of an Advertising Man: Selections from the Speeches, Articles, Memoranda, and Miscellaneous Writings of Leo Burnett”
Quoted in Reader's Digest, Sept. 1949. According to Nigel Rees, Cassell's Humorous Quotations, the following appeared in Punch in 1924: " 'It took me nearly ten years to learn that I couldn't write.' 'I suppose you gave it up then?' 'Oh, no! By that time I had a reputation established.' " The issue referred to by Rees is 6 Feb., and the cartoonist is R. Curry.
William Wordsworth, Dorothy Wordsworth, Ernest De Selincourt, Alan G. Hill, Chester Linn Shaver (1967). “The Letters of William and Dorothy Wordsworth: Volume VIII. A Supplement of New Letters”, p.51, Oxford University Press on Demand