A joke about “amen” and “awomen” and “hymns” and “hers” was printed in the Boston (MA) Transcript in September 1927 and reprinted in many newspapers:
   
“I wonder why they say ‘Amen’ and not ‘Awomen,’ Bobby.”
“Because they sing hymns and not hers, stupid.”
 
The joke has been told frequently. “Why do they say ‘amen’ and not ‘awomen’ at the end of songs at church? Because they’re hymns, not hers” was posted on Reddit—Dadjokes on June 14, 2022.
 
 
11 September 1927, The Sun (Baltimore, MD), pt. 2, sec. 3, pg. 2, col. 6:
MASCULINE WORLD, AGAIN.
“I wonder why they say ‘Amen’ and not ‘Awomen,’ Bobby.”
“Because they sing hymns and not hers, stupid.”—Boston Transcript.
         
Newspapers.com
12 September 1927, Indianapolis (IN) News, pg. 6, col. 6:
Bobby’s Explanation
[Boston Transcript]
“I wonder why they say ‘Amen’ and not ‘Awomen,’ Bobby.”
“Because they sing hymns and not hers, stupid.”
 
Google Books
September 1964, Boys’ Life, “Think and Grin,” pg. 74, col. 1:
Fred: Why do you say amen in a church instead of awomen?
Ted: I don’t know, why?
Fred: Because you sing hymns, not hers. — Randy Warren, Westbury, N.Y.
 
Google Books
Children in Search of Meaning:
A Study of Religious and Scientific Thought and Enquiry Arising from Experience in the Primary School Years
By Violet Madge
London, UK: SCM Press
1965
Pg. 72:
Roger
I’ll ask you another one , shall I ? Why do we sing Amen in church ?
Miss M.
Tell me why.
Roger
‘Cos we sing ‘hymns’ and not ‘hers’, see?
     
Google Books
Biggest Riddle Book in the World
By Joseph Rosenbloom
New York, NY: Sterling Publishing Company, Inc.
1976
Pg. 153:
Why do we say “amen” and not “awomen?”
Because we sing “hymns,” not “hers.”
 
Google Books
Doc Blakely’s Push Button Wit
By James Blakely
Houston, TX: Rich Publishing Company
1986
Pg. 206:
Libber to Preacher, “Why do you say Amen instead of Awoman?”
“Cause we sing hymns and not hers.”
 
Google Groups: soc.culture.indian
Desi jokes (A collection from some time ago)
Subramanyam Arunkumar
Apr 22, 1992, 5:39:59 PM
(...)   
A little girl and a boy are talking.
Girl : Why is it we always say “Amen” and not “Awomen” at the end of prayer ?
Boy : (Thinks for a while) It’s because we always sing “Hymns” and not “Hers”.
 
Google Groups: talk.origins
NP: David Schramm the Flim Flam Man, essay from Colo State
Ba Romaron
Sep 26, 1996, 3:00:00 AM
(...)
Here’s a riddle:
 
Q. Why do people in churches say “Amen”, not “Awomen”?
A. Because they sing “hymns”, not “hers”! 😊
   
Google Groups: soc.penpals
WHY DOES PEOPLE…..
Kalagan
Dec 10, 1997, 3:00:00 AM
Arne wrote in message <348D9167…@sk.sympatico.ca>

...
>
>
>+rash+ wrote:
>
>> .....ALWAYS ASK “IS GOD A MAN OR A WOMAN”????
>

>Because women do not realize that God is a man !~!
>

He’s a man for two reasons:
 
1) We say Amen instead of Awomen
2) We sing Hymns not Hers
   
Google Groups: alt.humor.puns
Prayer
pml
Mar 26, 1999, 3:00:00 AM
(...)
Why do people say Amen in church and not Awomen?
Because they sing hymns, not hers.
 
Reddit—Jokes
Posted by u/idpiasg October 16, 2013
Why do the say “Amen” in church but not “Awoman?”
Because they sing hymns, not hers!
 
Reddit—Dadjokes
Posted by u/Frontier_Justice October 10, 2015
At the end of prayers, why do people say ‘amen’ instead of ‘our women’?
Because they sing hymns, not hers.
     
Reddit—Jokes
Posted by u/danman1916 January 10, 2016
Why do they say “Amen”, not “Awomen”?
Because they sing “hymns” not “hers”
 
Reddit—Dadjokes
Posted by u/gevors_e92 June 14, 2022
Why do they say “amen” instead of “a woman” at the end of songs at church?
‘Cause they’re hymns not hers.
   
Twitter
Bob Kostic
@causticbob
Why do they say “amen” and not “awomen” at the end of songs at church?
Because they’re hymns, not hers.