“Those who say it cannot be done should not interrupt those who are doing it”

“Those who say it cannot be done should not interrupt those who are doing it” is a motivational sign that has been posted on the walls of many offices. The saying has been cited in print since at least 1985, but the authorship is unknown. Playwright George Bernard Shaw (1856-1950) is often given credit for the saying, but there is no evidence that he ever used it.
   
 
3 November 1985, Orlando (FL) Sentinel, “Inmates use talents to fix up buses” by Carolyn Cox, Lake Sentinel, pg. 1:
Motivational signs, such as “Those who say ‘it cannot be done’ should not interrupt those who are doing it,” are posted throughout the shop, which inmates…
     
Google Books
The Complete Book of Zingers
By Croft M. Pentz
Wheaton, IL: Tyndale House Publishers
1990
Pg. 298:
People who say that something is impossible should not interrupt those who are managing to get it done.
 
Google News Archive
3 August 1991, Eugene (OR) Register-Guard, “Embattled abortion doctor stays committed to women” by The Associated Press, pg. 8A, col. 1:
A hand-lettered sign in the corner of the office says: “Those who say it should not be done should not interrupt those who are doing it.”
     
Google Books
Chicken Soup for the Soul:
101 stories to open the heart & rekindle the spirit

By Jack Canfield and Mark Victor Hansen
Deerfield Beach, FL: Health Communications
1993
Pg. 145:
People who say it cannot be done should not interrupt those who are doing it.
   
Google Books
The Educator’s Book of Quotes
By John Blaydes
Thousand Oaks, CA: Corwin Press
2003
Pg. 74:
People who say it cannot be done should not interrupt those of us who are doing it. —George Bernard Shaw
 
Google Books
The Complete Guide to Building a Successful Trading Business
By Paul King
Middlebury, VT : PMKing Trading
2007
Pg. 11:
I live by the old Chinese philosophy that ‘people who say a thing is impossible should not interrupt those doing it’.