Samuel Butler (1613-1680) wrote in his English mock heroic narrative poem Hudibras (1678):
 
“He that complies against his will
Is of his own opinion still.”
 
“Men convinced against their will are of the same opinion still” was cited in print in 1819. “Those convinced against their will are of the same opinion still” was cited in 1822. American writer and lecturer Dale Carnegie (1888-1955) has frequently been credited with the line since the 1990s, but the verse had been popular long before Carnegie was born.
 
   
Wikipedia: Hudibras
Hudibras (/ˈhjuːdɪbræs/ or /ˈhjuːdɪbrəs/) is an English mock heroic narrative poem from the 17th century written by Samuel Butler.
 
Purpose
The work is a satirical polemic upon Roundheads, Puritans, Presbyterians and many of the other factions involved in the English Civil War. The work was begun, according to the title page, during the civil war and published in three parts in 1663, 1664 and 1678, with the first edition encompassing all three parts in 1684.
 
Wikiquote: Opinion
He that complies against his will
Is of his own opinion still.
. Samuel Butler, Hudibras, Part III (1678), Canto III, line 547.
   
Google Books
Noember 1819, The British Review and London Critical Journal, pg. 438:
It is a maxim trite but true, that
 
“Men convinced against their will
Are of the same opinion still;”
 
and this often holds good of individuals of superior intellect as well as of others; ...
 
9 December 1822, Alexandria (VA) Herald, “Newspaper Borrowers,” pg. 4, col. 2:
But as we think, with Hudibras, that
 
“Those convinced against their will
Are of the same opinion still,”
 
we shall not put our wits to work for naught.
 
Google Books
God on the Internet
By Mark A. Kellner
Foster City, CA: IDG Books Worldwide
1996
Pg. 31:
Some 70 years after Lincoln’s death, Dale Carnegie offered the view that “A man convinced against his will is of the same opinion still.”
 
Google Books
Alcoholic Relationship Survival Guide
By Doug Kelley and Tracy Kelley
Punta Gorda, FL: Kelley Training SYstems, Inc.
2006
Pg. 65:
Dale Carnegie wrote, “Those convinced against their will are of the same opinion still.”
 
Google Books
The Total Money Makeover:
A Proven Plan for Financial Fitness
By Dave Ramsey
Nashville, TN: Thomas Nelson
2009
Pg. XIII:
My grandmother used to say, “Those convinced against their will are of the same opinion still.”
 
Twitter
Personal Development
‏@persdevquotes
Those convinced against their will are of the same opinion still.  ~ Dale Carnegie http://bit.ly/persdev
6:35 PM - 28 Apr 2013