“Would you not think him an utter fool who wept because he was not alive a thousand years ago? And is he not just as much of a fool who weeps because he will not be alive a thousand years from now? It is all the same; you will not be, and you were not. Neither of these periods of time belongs to you.”

James Burgh, in The Dignity of Human Nature, Or, A Brief Account of the Certain and Established Means for Attaining the True End of Our Existence (1754); this is very widely misattributed to Mann, appearing at least as early as the publication of Thoughts Selected from the Writings of Horace Mann (1867) edited by Mary Mann.
Misattributed

Variant in other editions: Do not think of knocking out another person's brains, because he differs in opinion from you; it will be as rational to knock yourself on the head because you differ from what you thought ten years ago.
The Dignity of Human Nature (1754)

Gen. Edward Cummings, in Pt. 1, Ch. 3
The Naked and the Dead (1948)
Context: You're a fool if you don't realize this is going to be the reactionary's century, perhaps their thousand-year reign. It's the one thing Hitler said which wasn't completely hysterical.

Statement after the suicide of Aaron Swartz, in "An Incredible Soul": Larry Lessig Remembers Aaron Swartz After Cyberactivist’s Suicide Before Trial; Parents Blame Prosecutor" at Democracy NOW! (14 January 2013) http://www.democracynow.org/2013/1/14/an_incredible_soul_lawrence_lessig_remembers
Context: I received an email from JSTOR four days before Aaron died, from the president of JSTOR, announcing, celebrating that JSTOR was going to release all of these journal articles to anybody around the world who wanted access — exactly what Aaron was fighting for. And I didn’t have time to send it to Aaron; I was on — I was traveling. But I looked forward to seeing him again — I had just seen him the week before — and celebrating that this is what had happened. So, all of us think there are a thousand things we could have done, a thousand things we could have done, and we have to do, because Aaron Swartz is now an icon, an ideal. He is what we will be fighting for, all of us, for the rest of our lives. … Every time you saw Aaron, he was surrounded by five or 10 different people who loved and respected and worked with him. He was depressed because he was increasingly recognizing that the idealism he brought to this fight maybe wasn’t enough. When he saw all of his wealth gone, and he recognized his parents were going to have to mortgage their house so he could afford a lawyer to fight a government that treated him as if he were a 9/11 terrorist, as if what he was doing was threatening the infrastructure of the United States, when he saw that and he recognized how — how incredibly difficult that fight was going to be, of course he was depressed.
Now, you know, I’m not a psychiatrist. I don’t know whether there was something wrong with him because of — you know, beyond the rational reason he had to be depressed, but I don’t — I don’t — I don’t have patience for people who want to say, "Oh, this was just a crazy person; this was just a person with a psychological problem who killed himself." No. This was somebody — this was somebody who was pushed to the edge by what I think of as a kind of bullying by our government. A bullying by our government.

Source: Darkfever

You would be a fool to bat like Rohit Sharma, he is in a different class: KL Rahul, The Indian Express, 3 July 2019 https://indianexpress.com/article/sports/cricket-world-cup/rohit-sharma-kl-rahul-opening-india-vs-bangladesh-5812681/,
About him

Dogs
The Note-Books of Samuel Butler (1912), Part XIV - Higgledy-Piggledy

Source: The Long Dark Tea-Time of the Soul (1988), Ch. 3