“If you speak when angry, you’ll make the best speech you’ll ever regret” was credited to comedian Groucho Marx (1890-1977) in 1954. The line has also been credited to Henry Ward Beecher (1813-1887) and Ambrose Bierce (1842-1913), but there is no evidence that the saying existed before the 1950s.
 
 
3 November 1954, Greensboro (NC) Record, “Inside TV” by Eve Starr, pg. B-3, col. 4:
Groucho quips: “It takes a heap of spending to make a house a home.” His best advice to contestants is: “If you speak when angry, you’ll make the best speech you’ll ever regret.”
 
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The Reader’s Digest
Volume 69
1954
Pg. 140:
Speak when you’re angry and you’ll make the best speech you’ll ever regret. -Anonymous
 
25 September 1958, Lebanon (PA) Daily News, pg. 1, left masthead:
Speak when you are angry and you’ll make the best speech you’ll ever regret.
   
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The Complete Toastmaster:
A New Treasury of Speakers
By Herbert V. Prochnow
Englewood Cliffs, NJ: Prentice-Hall
1960
Pg. 55:
Speak when you are angry and you’ll make the best speech you’ll ever regret.
 
Google News Archive
9 October 1962, Beaver County Times (PA), pg. 6, col. 6:
Speak when you are angry, and you will make the finest speech you will ever regret. —Arcadia (Wis.) News-Leader.
 
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The Speaker’s Quote Book:
Over 5,000 Illustrations and Quotations for All Occasions
By Roy B. Zuck
Grand Rapids, MI: Kregel Academic & Professional
2009
Pg. 15:
Speak when you are angry and you’ll make the best speech you’ll ever regret. —Henry Ward Beecher
 
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Chicken Soup for the Soul:
All in the Family:
101 Incredible Stories about Our Funny, Quirky, Lovable & “Dysfunctional” Families
By Jack Canfield, Mark Victor Hansen, Amy Newmark and Susan M. Heim
New York, NY: Simon & Schuster
2009
Pg. ?:
Speak when you are angry and you will make the best speech you will ever regret. ~Ambrose Bierce