The motto of the Boy Scouts of America is “Be Prepared.” There’s a joke:
 
Q: What did the boy scout say when he fixed the car horn?
A: “Beep repaired.”
   
The joke has been cited in print since at least 1963. Boys’ Life (the monthly magazine of the Boy Scouts of America) has printed the joke many times since 1971.
 
   
Wikipedia: Scout Motto
The Scout Motto of the Scout movement, in various languages, has been used by millions of Scouts around the world since 1907. Most of the member organizations of the World Association of Girl Guides and Girl Scouts (WAGGGS) share this same motto.
(...)
Baden-Powell on “Be Prepared”
In English, this motto is most commonly Be Prepared.
     
16 July 1963, Cincinnati (OH) Enquirer, “Pigs Is Nervous!” by Ollie M. James, pg. 7, col. 2:
GORDON E> CORNELL of 6617 Thompson Rd. Zip Codes us with this: Hear about the guy with the little car? It had a little horn that went “Beep!” One day the horn quit working, and the man was trying to fix it when along came a Boy Scout who was electrically inclined.
 
Beep repaired.
 
As an honorary member of Brownie Troop No. 1 of Batavia, we don’ know what our motto is, but the Boy Scouts’ is “Be Prepared.” (We didn’t want you to puzzle all day over that “Beep Repaired.”)
 
Google Books 
September 1971, Boys’ Life, “Think and Grin, pg. 90, cols. 3-4:
The Scoutmaster barreled down the country road at night. SUddenly his headlights shown on a large cow in the middle of the road. He pushed on his horn, but all that came forth was a tiny, weak “beep!” He screeched to a halt, shooed the cow and lifted up the car’s hood. Within a few minutes he had finished. He returned to the driver’s seat. His wife asked him, “Did you get the horn fixed?” The Scoutmaster nodded and said, “Beep repaired.”—Robert Peyes, Shrub Oak, N.Y.
   
Google Books
June 1974, Boys’ Life, ‘Think and Grin,” pg. 74, col. 4:
Tenderfoot: What did the auto mechanic say to the Scoutmaster after fixing his horn?
Eagle: I give up, what?
Tenderfoot:  Beep repaired! — Bill Messenger, Seaford, Del.
 
Google Books
The Gigantic Joke Book
By Joseph Rosenbloom
Illustrations by Joyce Behr
New York, NY: Sterling Publishing Company, Inc.
1978
Pg. 229:
What did the Scoutmaster say when his car horn was fixed?
“Beep repaired.”
 
20 April 1980, Sunday Advocate (Baton Rouge, LA), “Jumble,” Magazine, pg. 22, col. 1:
What did the boy scout say after he fixed the horn on his bike—
“BEEP REPAIRED”
 
Google Books
Swine Lake:
Music & Dance Riddles
Compiled by Charles Keller
Illustrated by Gregory Filling
Englewood Cliffs, NJ: Prentice-Hall
1985      
Pg. ?:
What did the Boy Scout say when he fixed the horn on his bike?
“Beep repaired.”
 
Google Groups: rec.humor
need corny auto repair slogans    
.(JavaScript must be enabled to view this email address)
7/24/95
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Some boy scouts opened a garage that specialized in the repair of automobile horns.  Their motto:  “Beep Repaired”
 
Google Groups: alt.humor.puns
The Scoutmaster
Stan Kegel
8/18/99
This scoutmaster and his wife were driving along a rural highway, when they found the road blocked by a herd of cows that had escaped through a broken fence.
     
The scoutmaster tried honking his horn to scare the cattle from the pavement, but to no avail. For some reason, no sound was heard.
 
He got out of the car, lifted the hood, and saw the problem, a loose wire, which he quickly fixed. As he got back into the car, his wife asked him if he’d had any luck.
 
“Yep”, he replied. …  “Beep repaired!”
 
Twitter
Jeff Gill
‏@jqgill
Q: What do you call a horn that’s been fixed by a boy scout?
A: Beep repaired.
9:26 AM - 13 Jul 2009
 
Twitter
MOGGY
‏@DavidMoggy
What did the Boy Scout say after he fixed his dad’s car horn?
Beep repaired.
4:11 AM - 29 Dec 2014
 
Twitter
Mediocre Jokes.
‏@MediocreHumour
What did the boy scout say when he fixed the car horn? Beep repaired.
7:14 AM - 8 Sep 2015
 
reddit
My car horn hasn’t worked for a long time.
submitted July 26, 2016 by CaptainPunisher
Today, a Boy Scout fixed it and all he said was, “Beep repaired!”