“Sandry: "I am silly, now and then. My mother said I was, anyway."
Daja: "If you know, you can stop it."
Sandry: "Then you've never been silly or you'd know it just creeps up without any warning.”

Statement in his Introductory profile at The University of Adelaide http://www.eleceng.adelaide.edu.au/people/profiles/academic.html#abbott.

Reported in Hoyt's New Cyclopedia Of Practical Quotations (1922), p. 86. The quotation has been attributed to many others, such as Lord Chief Justice Campbell, Lord Chesterfield, Sir William Harcourt, Lord Pembroke, Lord Westbury, and to an anonymous judge, and said to have been spoken in court to Garter King at Arms, Rouge Dragon Pursuivant, or some other high-ranking herald, who had confused a "bend" with a "bar" or had demanded fees to which he was not entitled. George Bernard Shaw quotes it in Pygmalion (1912) in the form, "The silly people dont [sic] know their own silly business."

Maule cannot be the original source of the quotation, as it is quoted nearly twenty years before his birth in Charles Jenner's The Placid Man: Or, The Memoirs of Sir Charles Beville (1770): "Sir Harry Clayton ... was perhaps far better qualified to have written a Peerage of England than Garter King at Arms, or Rouge Dragon, or any of those parti-coloured officers of the court of honor, who, as a great man complained on a late solemnity, are but too often so silly as not to know their own silly business." "Old Lord Pembroke" (Henry Herbert, 9th Earl of Pembroke) is said by Horace Walpole (in a letter of May 28, 1774 to the Rev. William Cole) to have directed the quip, "Thou silly fellow! Thou dost not know thy own silly business," at John Anstis, Garter King at Arms (though in his 1833 edition of Walpole's letters to Sir Horace Mann, George Agar-Ellis, 1st Baron Dover, attributes the saying to Lord Chesterfield in a footnote, in the form "You foolish man, you do not understand your own foolish business"). Edmund Burke also quotes it ("'Silly man, that dost not know thy own silly trade!' was once well said: but the trade here is not silly.") in a "Speech in the Impeachment of Warren Hastings, Esq." on May 7, 1789 (when Maule was just over a year old). Chesterfield or Pembroke fit best in point of time.
Attributed

Source: Meditations in Wall Street (1940), p. 107

Quote from Turner's remark c. Jan, 1849, to financial agent Mr. Williams; as cited in 'The life of J.M.W. Turner', Volume II, George Walter Thornbury; https://ia801207.us.archive.org/18/items/lifeofjmwturnerr02thor/lifeofjmwturnerr02thor.pdf Hurst and Blackett Publishers, London, 1862, pp. 248-249
Mr. Drake, the solicitor of the Railway Company, whom Mr. Turner saw when he executed the conveyance, requested Mr. Williams to ask Turner's permission to show him a picture he had purchased as a 'Turner'
1821 - 1851

Interview, Science Fiction Review (August 1976)
Context: I think that, like in my writing, reality is always a soap bubble, Silly Putty thing anyway. In the universe people are in, people put their hands through the walls, and it turns out they're living in another century entirely. … I often have the feeling — and it does show up in my books — that this is all just a stage.

Estelle to Inès, Act 1, sc. 5
No Exit (1944)

MOJO interview (2005)
Context: For the last 12 years, I've felt really privileged to be living such a normal life. It's so a part of who I am. It's so important to me to do the washing, do the Hoovering. Friends of mine in the business don't know how dishwashers work. For me, that's frightening. I want to be in a position where I can function as a human being. Even more so now where you've got this sort of truly silly preoccupation with celebrities. Just because somebody's been in an ad on TV, so what? Who gives a toss?

Source: The Lightning Thief

Source: The Diary of a Young Girl