“I hereby state, and mean all I say, that I never have been and never will be a candidate for President; that if nominated by either party I should peremptorily decline; and even if unanimously elected I should decline to serve.”

Telegram sent to General Henderson in 1884, refusing to run in the United States presidential election of that year. As quoted in Sherman's Memoirs, 4th ed. 1891. This is often paraphrased: If nominated, I will not run; if elected, I will not serve.
1880s, 1884, Telegram (1884)

2016, DNC Address (July 2016)
Context: You know, nothing truly prepares you for the demands of the Oval Office. You can read about it. You can study it. But until you’ve sat at that desk, you don’t know what it’s like to manage a global crisis, or send young people to war. But Hillary has been in the room; she’s been part of those decisions. She knows what’s at stake in the decisions our government makes — what’s at stake for the working family, for the senior citizen, or the small business owner, for the soldier, for the veteran. And even in the midst of crisis, she listens to people, and she keeps her cool, and she treats everybody with respect. And no matter how daunting the odds, no matter how much people try to knock her down, she never, ever quits.
That is the Hillary I know. That’s the Hillary I’ve come to admire. And that’s why I can say with confidence there has never been a man or a woman — not me, not Bill, nobody — more qualified than Hillary Clinton to serve as president of the United States of America.

Source: Postscript to the Outsider (1967), p. 3

Letter declining the 1926 Pulitzer Prize for Arrowsmith

The Third Notebook: Part Two
No Longer Human

1960s, October surprise speech (1968)
Context: What we won when all of our people united just must not now be lost in suspicion, distrust, selfishness, and politics among any of our people. Believing this as I do, I have concluded that I should not permit the Presidency to become involved in the partisan divisions that are developing in this political year. With America's sons in the fields far away, with America's future under challenge right here at home, with our hopes and the world's hopes for peace in the balance every day, I do not believe that I should devote an hour or a day of my time to any personal partisan causes or to any duties other than the awesome duties of this office — the Presidency of your country. Accordingly, I shall not seek, and I will not accept, the nomination of my party for another term as your President.

In an interview to Luciana Gimenez broadcasted on 7 May 2019. Racism 'rare' in Brazil, says far right Bolsonaro https://www.france24.com/en/20190508-racism-rare-brazil-says-far-right-bolsonaro. France 24 (8 May 2019).

In John Sloan on Drawing and Painting. Mineola NY: Dover Publications, 2000. Originally published in 1939 as The Gist of Art, p. 7.
The Gist of Art (1939)

said of the Kennedy-Nixon TV debates in an unpublished manuscript, (dated 20 March 1961); Collecting Himself (1989).
From other writings