“Don't do anything stupid.”

Remarks quoted in Red Horizons: Chronicles of a Communist Spy Chief (1987) by Ion Mihai Pacepa, p. 110

Source: Carpe Jugulum

2010s, 2016, November, New York Times Interview (November 23, 2016)

Source: The Last Olympian

In George Bernard Shaw's Man and Superman, act II, there is the following dialogue:
TANNER: Let me remind you that Voltaire said that what was too silly to be said could be sung.
STRAKER. It wasn't Voltaire: it was Bow Mar Shay.
TANNER. I stand corrected: Beaumarchais of course.
This quote has also been attributed to Joseph Addison. In The Spectator, 21 March 1711 http://www.hoasm.org/VIIA/Spectator3-21-11.html, Addison wrote of "an establish'd Rule, which is receiv'd as such to this Day, That nothing is capable of being well set to Musick, that is not Nonsense."
Misattributed
Source: "Nowadays what isn't worth saying is sung" (Aujourd'hui ce qui ne vaut pas la peine d'être dit, on le chante) — Pierre de Beaumarchais, Le Barbier de Séville (1775), act I, scene II.

Source: The Son of Neptune

Source: Can You Keep a Secret?

As quoted by David Milner, "Kenpachiro Satsuma Interview III" http://www.davmil.org/www.kaijuconversations.com/satsum3.htm, Kaiju Conversations (December 1995)