In the paper Sur les représentations modulaire de degré 2 de Gal$(\overline{\mathbb{Q}}/\mathbb{Q})$, Serre makes the following comment:
Remarque. La relation existant entre "solutions de l'équation de Fermat" et "points de $p$-division de certaines courbes elliptiques" figure déjà dans un travail de Hurwitz ([20]) datant de 1886.
(Trans.: The connection between “Solutions of Fermat’s equation” and “points of order $p$ on certain elliptic curves” is already in work of Hurwitz from 1886)
Hurwitz’s paper is Über endliche Gruppen linearer Substitutionen, welche in der Theorie der elliptischen Transcendenten auftreten. I’m unable to read it myself. Can anyone shed light on Serre’s comment? How did Hurwitz connect rational points on high-genus curves to points of finite order on elliptic curves?