“Nobody has ever bet enough on a winning horse” is a popular saying at the racetrack, but it has also been used since the 1980s as an investment proverb. Robert Byrne’s book The 637 Best Things Anybody Ever Said (1982) credited “Nobody has ever bet enough on the winning horse” to sports gambling expert Jimmy “the GreeK” Snyder (1918-1996). However, Robert Byrne’s book 1,911 Best Things Anybody Ever Said (1988) credited “Nobody has ever bet enough on the winning horse” to “Overheard at a track by Richard Sasuly.”
 
Richard Sasuly (1913-2011) was a horseracing and gambling historian and has usually been credited with the remark. Byrne probably changed the credit from Jimmy the Greek to ““Overheard at a track by Richard Sasuly” based on additional credible information, but it’s not clear where the saying was published in Sasuly’s works.
 
 
Google Books
Bookies and Bettors:
Two Hundred Years of Gambling
By Richard Sasuly
New York, NY: Holt, Rinehart and Winston
1982
 
30 September 1982, Hutchinson (KS) News,  “Who said that?” by Dick West, pg. 4, cols. 1-2:
WASHINGTON (UPI)—In his new book, “The 637 Best Things Anybody Ever Said,” Robert Byrne includes at least 12 things he himself said.
(...)
“Nobody has ever bet enough on the winning horse.” (Jimmy the Greek)
   
Google News Archive
3 January 1986, Evening Herald (Rock Hill, SC), “Here’s what you can say in ‘86” by Ben Wattenberg, pg. 4, col. 4:
If the stock market keeps going up in 1986, you may quote Mr Anonymous: “Nobody has ever bet enough on the winning horse.”
 
15 February 1987, Kerrville (TX) Daily Times, “Investment Corner,” pg. 13A, col. 2:
I know that this is all hard to agree with when stock prices are rising. After all, nobody ever bet enough on a winning horse.
 
Google Books
1,911 Best Things Anybody Ever Said
By Robert Byrne
New York, NY: Fawcett, Columbine
1988
179
Nobody has ever bet enough on the winning horse.
Overheard at a track by Richard Sasuly
 
Google Books
Lightning Fast Enlightenment:
A Journey to the Secrets of Happiness
By Jordan S. Metzger
San Diego, CA: Profound Press
2000
Pg. 124:
Richard Sasuly: No one has ever bet enough on a winning horse.
 
Google Books
Winning Without Thinking:
A Guide to Horse Race Betting Systems
By Nick Mordin
Nantwich, Cheshire: Aesculus Press Limited
2003
Pg. 145:
“Nobody has ever bet enough on a winning horse.”
Richard Sasuly
   
Google Books
The Gigantic Book of Horse Wisdom
Edited by Thomas Meagher
New York, NY: Skyhorse Publishing, Inc.
2007
Pg. 474:
Nobody has ever bet enough on a winning horse. — AMERICAN PROVERB