“Surrounded by army, you think you're going to die. You can't shower because they'll find you nude.”

Saturday Night and Sunday Morning (1958; repr. New York: Alfred A. Knopf, 1959), p. 144.

Ensign Richard Sharpe to the Light Company of the 33rd Regiment of Foot, p. 261
Sharpe (Novel Series), Sharpe's Fortress (1999)

Source: Real Live Boyfriends: Yes. Boyfriends, Plural. If My Life Weren't Complicated, I Wouldn't Be Ruby Oliver

Tuesdays with Morrie (1997)

Credited to Shaw in the lead in to the mockumentary C.S.A.: The Confederate States of America (2004) and other recent works, but this or slight variants of it are also sometimes attributed to W. C. Fields, Charlie Chaplin, and Oscar Wilde. It might possibly be derived from Shaw's statement in John Bull's Other Island (1907): "My way of joking is to tell the truth. It's the funniest joke in the world."
Another possibility is that it is derived from Shaw's characteristic of Mark Twain: "He has to put things in such a way as to make people who would otherwise hang him believe he is joking."
Variants:
If you are going to tell people the truth, you'd better make them laugh. Otherwise, they'll kill you.
If you're going to tell people the truth, you'd better make them laugh. Otherwise, they'll kill you.
Disputed

Variant: If you think you can, you can. And if you think you can not, you are right.