This case frustrates me, because I saw this GIF posted a million times, but no one seems to know the source.
I remember that last year, or maybe in 2024, I searched for the source of the GIF and managed to find it. It was hosted on a site very similar to (if not identical to) Tumblr. There were lots of spicy cosplays, but in the background was the famous GIF in question, except that it was slower than the popular.

The only drawback is... I don't remember where I found the original GIF, and since I didn't know about this site, I reposted it on Tenor, a site that doesn't let you add notes or anything indicating where it came from.

I remember that the cosplayer's site had other gifs of Reisen & Tewi that aren't as popular as this one (and which I also saved).
Trust me, I tried to find the cosplayer's site again, but since Google implemented Gemini in its search engine, image searches have gotten significantly worse.

If anyone knows where these GIFs come from, let me know.