"Nobody who ever gave his best regretted it” has been credited to professional American football pioneer George Halas (1895-1983) since 1996. It’s not known when (or if) he said this during his lifetime.
Wikipedia: George Halas
George Stanley Halas, Sr. (February 2, 1895 – October 31, 1983), nicknamed “Papa Bear” and “Mr. Everything”, was a player, coach, owner and pioneer in professional American football. He was the iconic founder and owner of the National Football League’s Chicago Bears. He was also lesser known as an inventor, jurist, producer, philanthropist, philatelist, and Major League Baseball player.
A Hero in Every Heart:
Champions from all walks of life share powerful messages to inspire the hero in each of us
By H. Jackson Brown and Robyn Freedman Spizman
Nashville, TN: T. Nelson
Nobody who ever gave his best regretted it. — George Halas
Secrets of a CEO Coach:
Your Personal Training Guide to Thinking Like a Leader and Acting Like a CEO
By D. A. Benton
New York, NY: McGraw Hill Professional
“Nobody who ever gave his best regretted it.” said legendary Chicago Bears coach George Halas.
The 17 Essential Qualities of a Team Player:
Becoming the Kind of Person Every Team Wants
By John Maxwell
Nashville, TN: Thomas Nelson
George Halas, former owner of the NFL Chicago Bears, asserted, “Nobody who ever gave his best regretted it.”
Classic Quotes from the World of Sports
By Michael Benson
Lanham, MD: Taylor Trade Publishing
“Nobody who ever gave his best regretted it.” —George Halas