Socialism ― or the tyranny of the meanest and the most brainless, ―that is to say, the superficial, the envious, and the mummers, brought to its zenith, ―is, as a matter of fact, the logical conclusion of “modern ideas” and their latent anarchy: but in the genial atmosphere of democratic well-being the capacity for forming resolutions or even for coming to an end at all, is paralysed. Men follow―but no longer their reason. That is why socialism is on the whole a hopelessly bitter affair: and there is nothing more amusing than to observe the discord between the poisonous and desperate faces of present-day socialists―and what wretched and nonsensical feelings does not their style reveal to us! ―and the childish lamblike happiness of their hopes and desires. Nevertheless, in many places in Europe, there may be violent hand-to-hand struggles and irruptions on their account: the coming century is likely to be convulsed in more than one spot, and the Paris Commune, which finds defenders and advocates even in Germany, will seem to have but a slight indigestion compared with what is to come. Be this as it may, there will always be too many people of property for socialism ever to signify anything more than an attack of illness: and these people of property are like one man with one faith, “one must possess something in order to be some one.” This, however, is the oldest and most wholesome of all instincts; I should add: “one must desire more than one has in order to become more.” For this is the teaching which life itself preaches to all living things: the morality of Development. To have and to wish to have more, in a word, Growth―that is life itself. In the teaching of socialism “a will to the denial of life” is but poorly concealed: botched men and races they must be who have devised a teaching of this sort. In fact, I even wish a few experiments might be made to show that in socialistic society life denies itself, and itself cuts away its own roots. The earth is big enough and man is still unexhausted enough for a practical lesson of this sort and demonstratio ad absurdum― even if it were accomplished only by a vast expenditure of lives―to seem worth while to me. Still, Socialism, like a restless mole beneath the foundations of a society wallowing in stupidity, will be able to achieve something useful and salutary: it delays “Peace on Earth” and the whole process of character-softening of the democratic herding animal; it forces the European to have an extra supply of intellect, ―it also saves Europe awhile from the marasmus femininus which is threatening it.

☛ The Complete Works Friedrich Nietzsche, Vol XIV: The Will to Power : An Attempted Transvaluation of All Values, tr. by Anthony M. Ludovici and edited by Oscar Levy, Edinburgh and London: T.N. Foulis, § 125, p. 102-103. Internet Archive.

Given the controversial nature of The Will to Power (see in French La volonté de puissance by Paolo D’Iorio), the reference used above will eventually become obsolete: the quote should be referenced as a fragment of Nietzsche’s posthumous work. To my knowledge, a complete and critical edition of those posthumous fragments has yet to be edited in English. However, such an edition is already available in French and in German. Both are based on the critical text established by Giorgio Colli and Mazzino Montinari. The French version was edited by Gallimard: Oeuvres philosophiques complètes, vol. XI, tr. by Michel Haar and Marc de Launay, Paris, 1982. The German version exist online as a digital edition through Nietzsche Source: learn more about the Digital Critical Edition of Nietzsche’s Works and Letters (eKGWB).nIn the German Digital Critical Edition, the fragment about socialism quoted above is referenced as followed: NF-1885, 37[11].
Nietzsche wrote down those thoughts on the summer of 1885 (in June or July). It really is tempting to read what he says about the “vast expenditure of lives” as an accurate prediction of what various communist regimes of the twentieth century would eventually turn out to be accountable for.
I thought about Nietzsche’s comment on socialism while reading On the shores of politics by Jacques Rancière. In a chapter titled “The Community of Equals” the French philosopher discusses the use of “the standard of equality” as the main founding principle for the ”communitarian body”:

For it may well be that relations of community and equality are themselves but a never-ending settling of accounts. By taking a closer look at the accounts presented by equality to community we shall see the image of the single great body crumble, and encounter all the deficit and discord which ensure that the community of equals can never materialize without some cement plugging the cracks in the image, without some obligation to keep tallying members and ranks and retranslating the terms of the formula. (tr. by Liz Heron, London: Verso, [1990]1995, p. 65)

On the shores of politics was first published in French in 1990. A second edition revised and expanded was published in 1998. Below is the French version of Rancière’s quote as it appears in the 1998 edition:

Les rapports de l’égalité et de la communauté ne sont peut-être eux-mêmes qu’un incessant règlement de comptes. Regarder de plus près ces comptes de l’égalité avec la communauté, c’est voir se fragmenter l’image du grand corps, rencontrer le déficit ou le discord qui fait que la communauté des égaux ne peut jamais se donner corps sans quelque plâtrage, sans quelque obligation de recompter les membres et les rangs, de boucher les fissures de l’image, de retraduire les énoncés de la formule. (éd. Gallimard, coll. Folio, 1998, p. 132).

To be fair, I should make a note that Rancière’s project aims at questioning the relationship between the principle of equality and the community, whereas Nietzsche’s quote is essentially an irrevocable condemnation of socialism as an attempt to build a society on the ideal of equality.
See also Balzac on equality between human beings.