“If you tell the truth, you don’t have to remember anything” is a quotation that has been printed on many posters. The quotation is usually credited to Mark Twain, the pen name of Samuel Langhorne Clemons (1835-1910). The only evidence crediting Twain is Mark Twain’s Notebook (1935), edited by Albert Bigelow Paine, where the Twain entry is supposedly from 1894.
 
Similar sayings have been cited in print before 1935. “If you always tell the truth you will never have to fix up excuses” was cited in 1898. “Another good thing about telling the truth is that you don’t have to remember what you say” was cited in many newspapers in 1922.
 
   
Wikiquote: Mark Twain
Samuel Langhorne Clemens (November 30, 1835 – April 21, 1910), better known by his pen name Mark Twain, was an American humorist, novelist, writer, and lecturer.
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If you tell the truth you don’t have to remember anything.
. Notebook entry, January or February 1894, Mark Twain’s Notebook, ed. Albert Bigelow Paine (1935), p. 240
 
Chronicling America
23 October 1898, St. Paul (MN) Globe,  “Ironical Ifs,” pg. 16, col. 6:
From the Chicago News.
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If you always tell the truth you will never have to fix up excuses.
   
5 August 1922, Tampa (FL) Morning Tribune, pg. 4, col. 6 ad:
Another good thing about telling the truth is that you don’t have to remember what you say.
(Turner Music Co.—ed.)
 
Chronicling America
21 November 1922, Bemidji (MN) Daily Pioneer, pg. 6, col. 1:
Another Good Thing
Health Hint: Another good thing about telling the truth is that you don’t have to remember what you say.
       
14 July 1924, Aberdeen (SD) Evening News, pg. 4, col. 6 ad:
Tell the Truth, and You Don’t Have to Remember what You Say
(First State Savings Bank.—ed.)
 
3 December 1924, The Evening Repository (Canton, OH), pg. 4, col. 3:
No Tax On Memory
Mr. Goode—My boy, why is it always best to tell the truth?
Boy—Because you don’t have to remember what you say.—Philadelphia Bulletin.
 
OCLC WorldCat record
Mark Twain’s notebook
Author: Mark Twain; Albert Bigelow Paine
Publisher: New York ; London : Harper & Brothers Publishers, MCMXXXV [1935]
Edition/Format:   Print book : English : First edition
 
15 October 1935, Greensboro (NC) Daily News, “Mark Twain’s Notebook” by Albert Bigelow Paine (“The First Reader” review by Harry Hanson), pg. 6, col. 6:
If you tell the truth you don’t have to remember anything.
 
25 November 1945, Boston (MA) Herald,“Twain Wit” by Will Yolen, This Week Magazine, pg. 2, col. 1:
“If you tell the truth, you don’t have to remember anything.”
 
Google Books
Mark Twain at Your Fingertips
Edited by Caroline Thomas Harnsberger
New York, NY: Beechhurst Press, Inc.
1948
Pg. 484:
If you tell the truth you don’t have to remember anything.
P. 240 — Mark Twain’s Notebook [1935 ed.]
 
Google Books 
14,000 Quips & Quotes:
For Speakers, Writers, Editors, Preachers, and Teachers
By E. C. McKenzie
Grand Rapids, MI: Baker Book House
1990, ©1980    
Pg. 522:
Another good thing about telling the truth — you don’t have to remember what you said.
 
Grammarphobia
Quote magnets
JANUARY 14TH, 2013
Q: I enjoyed hearing Pat discuss quote magnets last month on WNYC. I have a favorite Mark Twain quote, and I’d like to know whether it’s genuine: “If you always tell the truth, you don’t have to remember anything.”
 
A: We can’t find any evidence that Mark Twain ever wrote this. We can’t find it in any of his works, and the Internet websites that say he wrote it don’t say where.
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We found an early version—“If you always tell the truth, you will never have to fix up excuses”—among a list of anonymous “Ironical Ifs” printed in the Bay City (Michigan) Times-Press on Nov. 19, 1898. The same list was reprinted the following year in the Muskegon (Michigan) Chronicle.
 
Twain was alive and kicking then and was wildly popular. If those newspapers had been quoting him, they no doubt would have said so.
 
Land of Inspiration
30 Unknown Inspirational Quotes That Will Help You In Life.
27/05/2015 BY YASH BIRAJDAR
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13) “If you tell the truth, you don’t have to remember anything.”