A penicillin joke was told in 1959 by an unknown jokester:
 
“Son, there’s got to be a future for you. After all, they make penicillin out of moldy bread!”
 
Bronx-born comedian Milt Moss told Parade magazine in 1962 that his mother cheered him up: “Don’t worry about success. If they can make penicillin out of moldy bread, you can surely make something out of yourself.”
 
The jocular saying is usually credited to American boxer Muhammad Ali (1942-2016). Ali told a student at New York City’s New School of Social Research in 1979:
   
“Stay in college, get the knowledge. Stay there till you’re through. If they can make penicillin out of moldy bread, they can sure make something out of you.”
     
 
Google News Archive
21 August 1959, Milwaukee (WI) Sentinel, “Man About Milwaukee” by Buck Herzog, pt. 1, pg. 15, col. 1:
Have a jollie for you: “Son, there’s got to be a future for you. After all, they make penicillin out of moldy bread!”
   
Google Books
The Geochemical News
Issues 20-39
1960
Pg. 67:
Sand-in-the-Gears-of-Learning Department
From the Professor:
“If they can make penicillin out of moldy bread, I can surely make something out of you.”
 
Google News Archive
11 November 1962, Reading (PA) Eagle, “My Favorite Jokes” by Milt Moss, Parade magazine, pg. 22, col. 3:
Whenever I’m discouraged, my mother cheers me up by saying: “Don’t worry about success. If they can make penicillin out of moldy bread, you can surely make something out of yourself.”
 
24 July 1977, Boston (MA) Globe, “My Favorite Jokes” by Leonard Barr, Parade magazine, pg. 19, col. 3:
My manager sometimes gives me a little encouragement. She said, “If they can make penicillin out of moldy bread, there must be something they can do with you.”
 
22 November 1979, New York (NY) Times, “A Lecture by Muhammad Ali: The Topic Is the Greatest” by Dena Kleiman, pg. B3, cols. 1-2:
THese were the graduates of Muhammad Ali 101, a seven-week course at the New School for Social Research that cost $60 and ended Monday night wih an appearance by the 37-year-old retired heavyweight champion.
(...)
“What do you advise a black youth like myself?” someone asked.
 
“Stay in college, get the knowledge,” Mr. Ali replied. “Stay there till you’re through. If they can make penicillin out of moldy bread, they can sure make something out of you.”
 
Google Books
3 January 1980, Jet magazine, “Word of the Week,” pg. 32:
Muhammad Ali, the dynamic 6-foot-2, 240-pound retired heavyweight boxing champion, addressing students at New York City’s New School for Social Research on the need for a college education: “Stay in college, get the knowledge. Stay there ‘til you’re through. If they can make penicillin out of moldy bread, they can sure make something out of you.”
 
Google Books
Inspiration & Motivation
By Alfred Armand Montapert
Englewood Cliffs, NJ: Prentice-Hall
1982
Pg. 73:
YOUR POTENTIAL
“If they can make penicillin out of moldy bread, they can sure make something out of you.” — Muhammad Ali
 
Twitter
Todd Grisham
‏@GrishamESPN
“If they can make penicillin out of moldy bread they can surely make something out of you” - Muhammad Ali.  RIP champ.
11:28 PM - 3 Jun 2016
Southington, CT
   
Business Insider
22 of Muhammad Ali’s greatest quotes of all time
Mallory Schlossberg
June 4, 2016
(...)
“If they can make penicillin out of moldy bread, they can sure make something out of you.”