“Beware the wolf in sheep's clothing.”

Source: The Enemy

On Ramsay MacDonald. This is often taken as referring to Clement Attlee, but Scottish historian D. W. Brogan is cited in Safire’s Political Dictionary (2008), William Safire, Oxford University Press US, p. 352 ISBN 0195343344 as follows: ‘Sir Winston Churchill never said of Clement Attlee that he was a sheep in sheep’s clothing. I have this on the excellent authority of Sir Winston himself. The phrase was totally inapplicable to Mr. Attlee. It was applicable, and applied, to J. Ramsay MacDonald, a very different kind of Labour leader.’
Early career years (1898–1929)

Edmund Gosse, quoted in Ferris Greenslet Under the Bridge: An Autobiography (Boston: Houghton Mifflin, 1943) p. 104.
Sometimes misattributed to Yeats.
Criticism

Of T. Sturge Moore, c. 1906
Quoted in Ferris Greenslet, Under the Bridge, ch.12.

Morrison v. Olsen, 487 U.S. 654, 699 (1988) (dissenting).
1980s

Jacula Prudentum (1651)

Jacula Prudentum (1651)

Referring to Second-order logic, in Philosophy of Logic (1970)
1970s