Source of select quotes: Wikipedia.
"A commodity is... a mysterious thing, simply because in it the social character of men’s labour appears to them as an objective character stamped upon the product of that labour; because the relation of the producers to the sum total of their own labour is presented to them as a social relation, existing not between themselves, but between the products of their labour. This is the reason why the products of labour become commodities, social things whose qualities are at the same time perceptible and imperceptible by the senses. In the same way the light from an object is perceived by us not as the subjective excitation of our optic nerve, but as the objective form of something outside the eye itself. But, in the act of seeing, there is at all events, an actual passage of light from one thing to another, from the external object to the eye. There is a physical relation between physical things. But it is different with commodities. There, the existence of the things qua commodities, and the value relation between the products of labour which stamps them as commodities, have absolutely no connection with their physical properties and with the material relations arising therefrom. There it is a definite social relation between men, that assumes, in their eyes, the fantastic form of a relation between things. In order, therefore, to find an analogy, we must have recourse to the mist-enveloped regions of the religious world. In that world the productions of the human brain appear as independent beings endowed with life, and entering into relation both with one another and the human race. So it is in the world of commodities with the products of men’s hands. This I call the Fetishism which attaches itself to the products of labour, so soon as they are produced as commodities, and which is therefore inseparable from the production of commodities. This Fetishism of commodities has its origin, as the foregoing analysis has already shown, in the peculiar social character of the labour that produces them." - Karl Marx, Capital Vol. 1, chaper 1 section 4
"The savages of Cuba regarded gold as a fetish of the Spaniards. They celebrated a feast in its honour, sang in a circle around it and then threw it into the sea. If the Cuban savages had been present at the sitting of the Rhine Province Assembly, would they not have regarded wood as the Rhinelanders' fetish? But a subsequent sitting would have taught them that the worship of animals is connected with this fetishism, and they would have thrown the hares into the sea in order to save the human beings." - 1842 articles in the Rheinische Zeitung about Debates on the Law on Thefts of Wood.
"The nations which are still dazzled by the sensuous glitter of precious metals, and are therefore still fetish-worshippers of metal money, are not yet fully developed money-nations. Contrast of France and England. The extent to which the solution of theoretical riddles is the task of practice and effected through practice, the extent to which true practice is the condition of a real and positive theory, is shown, for example, in fetishism. The sensuous consciousness of the fetish-worshipper is different from that of the Greek, because his sensuous existence is different. The abstract enmity between sense and spirit is necessary so long as the human feeling for nature, the human sense of nature, and therefore also the natural sense of man, are not yet produced by man’s own labour. - Karl Marx, "Economic and Philosophic Manuscripts of 1844", in Marx-Engels Collected Works, Vol. 3. Moscow: Progress, 1975, p. 312).
"In real history, wage labour arises out of the dissolution of slavery and serfdom --or of the decay of communal property, as with oriental and Slavonic peoples -- and, in its adequate, epoch-making form, the form which takes possession of the entire social being of labour, out of the decline and fall of the guild economy, of the system of Estates, of labour and income in kind, of industry carried on as rural subsidiary occupation, of small-scale feudal agriculture etc. In all these real historic transitions, wage labour appears as the dissolution, the annihilation of relations in which labour was fixed on all sides, in its income, its content, its location, its scope etc. Hence as negation of the stability of labour and of its remuneration. The direct transition from the African's fetish to Voltaire's supreme being, or from the hunting gear of a North American savage to the capital of the Bank of England, is not so absurdly contrary to history as is the transition from Bastiat's fisherman to the wage labourer. - Karl Marx, Grundrisse, chapter 17 (1857).
"...we find in the capitalist process of production [an] indissoluble fusion of use-values in which capital subsists [as] means of production and objects defined as capital, when what we are really faced with is a definite social relationship of production. In consequence the product embedded in this mode of production is equated with the commodity by those who have to deal with it. It is this that forms the foundation for the fetishism of the political economists" - Karl Marx,Results of the Immediate Process of Production, appendix in Capital Volume 1. Penguin edition, 1976, p. 983).