Did All Might Really Say “Remember for What Cause You Clench Your Fists”?

Yes. All Might says "if you feel yourself hitting up against your limit, remember for what cause you clench your fists" in My Hero Academia, season 3, episode 11, and the second half that usually rides with it, about remembering why you started down this path and letting the memory carry you beyond your limit, is his too. The words on the poster are the dub's.

The verdict

True. Screen Rant's ranking of All Might's best lines places it in season 3, episode 11, "One For All," and CBR's collection of the series' lines about heroism prints it as well. My Hero Academia has no Wikiquote page, so this is a line that rests on the editorial tier, as every line from the series on this site does; two publishers agreeing on the episode, in body text rather than a heading, is as good as the record for this show gets. The scene is the one the episode is named for.

Where it sits in the story

It is not a pep talk. All Might is in the middle of his last fight, against All For One in Kamino, with the embers of his power almost gone, and the words are addressed to Deku through the memory of what he has been teaching him: this is what you do when the body is finished and the fight is not. Then he does it. The line is the thesis of the punch that follows, the one that ends All For One and ends All Might as a hero, and the "you are next" he points at the camera afterwards is the other half of the same thought. On a poster the sentence reads as a gym slogan about limits. In the scene it is a man spending the last of himself on purpose, explaining the purpose while he does it.

Why it travels

Because it is genuinely built for travelling; Horikoshi writes All Might in slogans on purpose, since a symbol of peace is a man whose job is to be quotable. That is why the series' fabricated lines are rare compared with Naruto's or Dragon Ball's: the real ones already sound like posters. The risk with All Might is not invention but flattening, and this line is the clearest case, because the context that makes it more than motivation is the one thing the image cannot carry. This site's My Hero Academia fabrications, Momo's bright future and Kirishima's power to smile, were both invented in exactly All Might's register, which is the compliment the forgers paid him.

What sits beside it

All Might's other lines, and Deku's, are on the My Hero Academia hub, each with the editorial source it came from; where the series stands now, and what became of the man who said this, is in the ending explainer.

How we checked

The verification process asks for a primary or editorial source; here there are two editorial ones from different publishers, agreeing on the episode. The entry in the fact-check database is true, listed because the line's poster life has detached it from a scene that is worth putting back.