Stock investor John Templeton (1912-2008) is credited with giving this investment wisdom in February 1994: “Bull markets are born on pessimism, grow on skepticism, mature on optimism, and die on euphoria. The time of maximum pessimism is the best time to buy, and the time of maximum optimism is the best time to sell.”
 
It’s not known if Templeton invented this in 1994 or had used this saying earlier. A Personal Finance book published in 1991 (the Google Books date might be incorrect) appears to have something very close to Templeton’s saying: “The stock market cycle of investor thinking goes through four stages: pessimism, skepticism, optimism, and euphoria.”
 
 
Wikipedia: John Templeton
Sir John Marks Templeton (November 29, 1912 – July 8, 2008) was an American-born British stock investor, businessman and philanthropist.
   
Google Books
Personal Finance
Third Edition
By E. Thomas Garman and Raymond E. Forgue
Boston, MA: Houghton Mifflin Co.
1991
Pg. 691:
The stock market cycle of investor thinking goes through four stages: pessimism, skepticism, optimism, and euphoria.
 
9 May 1994, Atlanta (GA) Journal-Constitution, “Templeton offers investment tips,” pg. E5:
The legendary Sir John Templeton has made a fortune picking stocks for himself and others who invested in the mutual funds that still bear his name.
(...)
“Bull markets are born on pessimism, grow on skepticism, mature on optimism and die on euphoria.”
       
Business Edge News Magazine
Calgary boasts biggest hat in small caps
By Gyle Konotopetz - Business Edge
Published: 02/06/2003 - Vol. 3, No. 6
(...)
SAGE WORDS: “Bull markets are born on pessimism, grow on skepticism, mature on optimism and they die on euphoria. The time of maximum pessimism is the best time to buy and the time of maximum optimism is the best time to sell.”
- Investing legend Sir John Templeton, Feb. 1954.
 
Google Books
Investing the Templeton way:
The market-beating strategies of value investing’s legendary bargain hunter
By Lauren C. Templeton and Scott Phillips
New York, NY: McGraw-Hill
2008
Pg. 23:
Bull markets are born on pessism, grow on skepticism, mature on optimism, and die on euphoria. The time of maximum pessimism is the best time to buy, and the time of maximum optimism is the best time to sell.
—Sir John Templeton, February 1994
   
The Motley Fool
5 Stocks Losing Top Investor Support
By Motley Fool Staff
January 28, 2009
“Bull markets are born on pessimism, grow on skepticism, mature on optimism, and die on euphoria. The time of maximum pessimism is the best time to buy, and the time of maximum optimism is the best time to sell.”—Sir John Templeton
 
Google Books
Stock Trader’s Almanac 2011
By Jeffrey A. Hirsch
Hoboken, NJ: John Wiley
2010
Pg. 49:
Bull markets are born on pessimism, grow on skepticism, mature on optimism, and die on euphoria. — Sir John Templeton (Founder, Templeton Funds, philanthropist, 1994)