American poet Robert Frost (1874-1963) told WQED, as reported in Collier’s Illustrated Weekly in 1956:
 
“People have got to think. Thinking isn’t to agree or disagree. That’s voting.”
 
The saying has been included in many quotation collections.
 
   
Wikipedia: Robert Frost
Robert Lee Frost (March 26, 1874 – January 29, 1963) was an American poet. He is highly regarded for his realistic depictions of rural life and his command of American colloquial speech.[1] His work frequently employed settings from rural life in New England in the early twentieth century, using them to examine complex social and philosophical themes. One of the most popular and critically respected American poets of his generation, Frost was honored frequently during his lifetime, receiving four Pulitzer Prizes for Poetry.
 
Google Books
Collier’s Illustrated Weekly
Volume 137
1956
Pg. 44:
For the first wise man of its series, WQED chose a poet — New England’s Robert Frost.
(...)
ON BEING YOURSELF
People have got to think. Thinking isn’t to agree or disagree. That’s voting.
 
26 May 1960, The Berkshire Eagle (Pittsfield, MA)< pg. 22, col. 6:

People have got to think. Thinking isn’t to agree or disagree—that’s voting.”—Robert Frost, poet.
 
Google Books
A Speaker’s Treasury for Educators, Convocation Speakers, Baccalaureate Speakers, Commencement Speakers, PTA Officers, School Board Members, and Others
By Herbert V. Prochnow
Grand Rapids, MI: Baker Book House
1973
Pg. 81:
People have got to think. Thinking isn’t to agree or disagree. That’s voting.
Robert Frost
 
Google Books
20,000 Quips & Quotes
By Evan Esar
New York, NY: Barnes & Noble Books
1995, ©1968
Pg. 808:
Thinking isn’t to agree or disagree — that’s voting. — Robert Frost
 
Google Books
Your Vote Is Magic!
By Lyn Dillies
Westport, MA: Lovebug Publishing
2012
Pg. ?:
Thinking is not to agree or disagree. That is voting.
Robert Frost