“Imitation is the sincerest form of flattery” is a saying by English writer Charles Caleb Colton  (1780-1832). “Immigration is the sincerest form of flattery” is a variation that has been printed on many images. The saying was printed in Life (a New York City-based humor magazine) on July 8, 1920.
   
Jack Paar (1918-2004), host of television’s The Tonight Show from 1957 to 1962, has frequently been credited with the saying since at least 1988. If Paar did say it, he certainly did not originate a saying that had already been popular.
   
   
8 July 1920, Life magazine (New York, NY), pg 70, col. 1:
IMMIGRATION is the sincerest form of flattery.
 
14 May 1925, Whitewright (TX) SUn, pg. 8, col. 6:
AIN’T IT
Whiz—“Lots of foreigners are coming into our country.”
Bang—“Yes, immigration is the sincerest form of flattery.”
   
29 September 1927, Life magazine, pg. 20, col. 1:
CONSOLATORY thought for Uncle Sam—Immigration is the sincerest form of flattery.
 
2 April 1929, Boston (MA) Globe, “Batch of Smiles,” pg. 28, col. 2:
“Immigration is the sincerest form of flattery.”—Los Angeles Times.
 
Google Books
The World of Oneness
By Byram Campbell
New York, NY: Vantage Press
1956
Pg. 150:
Chamberlin points out that immigration is the sincerest form of flattery, and that there has been a tidal wave of movement from countries under Communistic rule, as against a negligible trickle toward such areas (18).
     
Google Books
February 1959,

, pg. 2, col. 3:
America is proof that immigration is the sincerest form of flattery.
 
20 November 1988, Star Tribune (Minneapolis, MN), “A Baker’s dozen planks for U.S. foreign policy” by Harlan Cleveland, pg. 37A, col. 4:
Jack Paar, no foreign policy expert, said it: “Immigration is the sincerest form of flattery.”
 
Google Books
The 2,548 Best Things Anybody Ever Said
Selected and compiled by Robert Byrne
New York, NY: Fireside
2003
Pg. ?:
1,163
Immigration is the sincerest form of flattery. —Jack Paar
 
Twitter
Steve’s Quotes Today‏
@todayquotes
“Immigration is the sincerest form of flattery.” 
Jack Paar
wb 99 today
11:08 PM - 1 May 2017