He mentioned it in La Vie Errante (The Wandering Life), published 1890.
I found a quote in this blog post, both in French and in Amanda Crawley Jackson's English translation:
J’ai quitté Paris et même la France, parce que la tour Eiffel finissait par m’ennuyer trop.
Non seulement on la voyait de partout, mais on la trouvait partout, faite de toutes les matières connues, exposée à toutes les vitres, cauchemar inévitable et torturant. Ce n’est pas elle uniquement d’ailleurs qui m’a donné une irrésistible envie de vivre seul pendant quelque temps, mais tout ce qu’on a fait autour d’elle, dedans, dessus, aux environs.
Comment tous les journaux vraiment ont-ils osé nous parler d’architecture nouvelle à propos de cette carcasse métallique, car l’architecture, le plus incompris et le plus oublié des arts aujourd’hui, en est peut-être aussi le plus esthétique, le plus mystérieux et le plus nourri d’idées ? Il a eu ce privilège à travers les siècles de symboliser pour ainsi dire chaque époque, de résumer, par un très petit nombre de monuments typiques, la manière de penser, de sentir et de rêver d’une race et d’une civilisation. Quelques temples et quelques églises, quelques palais et quelques châteaux contiennent à peu près toute l’histoire de l’art à travers le monde, expriment à nos yeux mieux que des livres, par l’harmonie des lignes et le charme de l’ornementation, toute la grâce et la grandeur d’une époque.
Mais je me demande ce qu’on conclura de notre génération si quelque prochaine émeute ne déboulonne pas cette haute et maigre pyramide d’échelles de fer, squelette disgracieux et géant, dont la base semble faite pour porter un formidable monument de Cyclopes et qui avorte en un ridicule et mince profil de cheminée d’usine.
I left Paris and even France because in the end, the Eiffel Tower annoyed me too much.
Not only could you see it from wherever you went in the city, but you also found it everywhere, made in every material known to man, on sale in all the shop windows, an unavoidable and agonising nightmare. It wasn’t only the Eiffel Tower, however, that gave me an irresistible desire to live alone for a while, but everything that was done around, inside, above and adjacent to it.
Really – how could all the newspapers speak to us of a new architecture in relation to this metallic carcass, because architecture, the least understood and the most forgotten of the arts today is perhaps also the most aesthetic, the most mysterious and the most nourished with ideas. It has had the privilege, across the centuries, of symbolizing as it were each age, of summarizing in a very small number of typical monuments a race and a civilisation’s way of thinking, feeling and dreaming. A few temples and churches, palaces and châteaux contain more or less the world’s entire history of art, and express visually, better than books, through the harmony of lines and the charm of ornamentation all the grace and grandeur of an epoch.
But I wonder what will be concluded of our generation if some future riot does not topple this tall, skinny pyramid of iron ladders, this ungainly, giant skeleton whose base appears designed to carry a formidable monument of the Cyclops and which aborts in a ridiculous, thin profile of a factory chimney.
I couldn't find much information about this book online in English, but it has a French Wikipedia page which describes it as a récit de voyage (travelogue), a genre perhaps more non-fiction than fiction, but still written in a style intended to entertain.
It's not mentioned in any of his short stories.
I checked the complete Maupassant short stories available on Project Gutenberg (which even includes some non-Maupassant stories), but searching the text for "Eiffel" produced no results, and even searching for "tower" didn't give anything which could refer to the Eiffel Tower (mostly rural church towers and the like).
