“Libertarianism is rejected by the modern left — which preaches individualism but practices collectivism. Capitalism is rejected by the modern right-which preaches enterprise but practices protectionism.”

Source: Treason of the Intellectuals (1927), p. 151

Asinaria, Act III, scene 3, line 54 (line 644 of full Latin text).
Variant translation: Do you then yourself do that which you would be suggesting to us to do. (translator Henry Thomas Riley, 1912)
Asinaria (The One With the Asses)

Source: Just a Theory: Exploring the Nature of Science (2005), Chapter 2, “Just a Theory: What Scientists Do” (p. 24)

Deutsche Gottglaube, quoted in W. W. Coole (ed.), Thus Spake Germany (London: George Routledge & Sons, 1941), p. 9

Source: An Essay on Marxian Economics (Second Edition) (1966), Chapter VII, The Orthodox Theory of Profit, p. 57-58

As quoted in The Reader's Digest Vol. 135 (1989), p. 34
Variant: One man practicing sportsmanship is better than a hundred teaching it.

Quoted in Elitism or Populism: Pick Your Poison http://econlog.econlib.org/archives/2006/11/elitism_or_popu.html, by Arnold Kling (November 12, 2006)

Humility.
Table Talk (1689)

“English Aphorists,” p. 123
Reperusals and Recollections (1936)