He literally said he was gonna:
"Trump vows massive new tariffs if elected, risking global economic war"
https://www.washingtonpost.com/business/2023/08/22/trump-tra...
(https://archive.is/20231125045858/https://www.washingtonpost...)
EDIT - Found this after my post, a MUCH better "he said it":
https://www.donaldjtrump.com/agenda47/agenda47-president-tru...
No, knowing that Trump really likes tariffs is not enough to know specifically how he's going to do it. (And which laws he's going to break to get there.)
Well yeah, but the man is also a pathological liar. I would not blame anyone for not believing he was going to do anything that he said he would do.
And he did it last time too: https://en.wikipedia.org/wiki/Tariffs_in_the_first_Trump_adm...
“Living under a rock” is the technical term, I believe.