"Like riding a bicycle” is an expression meaning that once something is learned, it is not forgotten. “It’s like riding a bicycle, You never forget” was cited in print in 1917. “"They say it’s like riding a bicycle. Once you learn how, you never forget it” was cited in print in 1933.
Wiktionary: like riding a bike
. like riding a bicycle
like riding a bike
1. (simile) Said of skill that, once learned, is never forgotten.
November 1917, Boys’ Life, pg. 6, col. 2:
“It’s like riding a bicycle,” Ritter argued. “You never forget.”
31 January 1932, Waterloo (IA) Sunday Courier, “Riding a Bicycle,” pg. 10, col. 7:
There are lots of other things much like riding a bicycle. There’s a “knack” to doing them. When you are trying to learn, the task seems terribly hard, but once you have conquered it, it’s as easy as pie. For this reason, you should never give up easily, no matter what you are trying to learn. There are “knacks” about doing school work and other things, just as there are about riding a bicycle. Stick to your job until you acquire these knacks, and then you will never forget them.
7 May 1933, Springfield (MA) Sunday Union and Republican, “Trouble with Roller Skates Is They Roll” by Weare Holbrook, pg. 5E, col. 7:
“They say it’s like riding a bicycle,” I muttered encouragingly. “Once you learn how, you never forget it.”
Something Short and Sweet
By Herbert Ernest Bates
“Shorthand’s like riding a bike. Once you can you always can.”
April 1939, Boys’ Life, pg. 16, col. 3:
Learning the code is like riding a bike — once you know it you will never forget it—you may lose speed but you will never forget.
26 August 1956, New York (NY) Times, “Fans here laud spry old-timers” by Ernest Sisto, pg. S1:
“See it s like riding a bicycle. Once you learn how, you never forget,” said Robbie as he strode majestically back to the dugout.
January 1958, Popular Mechanics, pg. 150, col. 1:
Firing the Ml must be like riding a bicycle — once learned, it’s not forgotten.
Google News Archive
28 May 1974, St. Joseph (MO) News-Press, “Cards’ Curtis Finds Mark Against Dodgers,” pg. 2B, col. 5:
“It’s not like riding a bike, something that once you learn you don’t forget,” he cautioned, however.