Speak when you are angry and you will make the best speech you will ever regret.

Groucho Marx (1890-1977) American comedian [b. Julius Henry Marx]
(Attributed)

Quoted by Ever Star, "Inside TV," Greensboro Record (3 Nov 1954). Also attributed to Ambrose Bierce, Henry Ward Beecher, and Lawrence J. Peter. More research and discussion here.