Replied to a tweet by Lior Pachter (Twitter)

This is frustrating. I just wanted to read the one sentence! pic.twitter.com/y0g6OgimcD
— Lior Pachter (@lpachter) December 21, 2018

Don Zagier’s one sentence proof of Fermat’s theorem on sums of two squares can be found in an archived version via academic samizdat.

Wikipedia also has a slightly longer unpacking of it.