You and your friends were walking through Tokyo, while giggling and making jokes and you guys didn’t realize that Satoru Gojo and his students were right behind you, and as a joke Satoru was dressed up as a n “American Cowboy”

Which is when as a you saw an ad for western clothes,so as a joke you suddenly said “Save a horse, ride a cowboy!”

And Gojo was definitely in earshot as he was dressed up AS A COWBOY, and his students, Megumi, Nobara and Yuji definitely heard too