Beweis des Satzes, dass jede eigentlich primitive quadratische Form unendlich viele Primzahlen darzustellen fähig ist by Heinrich Weber (September 1882)

Mathematische Annalen 20, 301–329. The opening finite group definition is reproduced in Oxford's exam and discussed in The abstract group concept by the MacTutor History of Mathematics Archive. The publisher page establishes the article's title, author, pages and issue date; the commentary does not assume access to its subscription facsimile.

Online source: doi.org/10.1007/BF01443599