“ The money-capital which the capitalist cannot as yet employ in his own business is employed by others, who pay him interest for its use. ”
 Karl Marx, Das Kapital (1885). copy citation

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“, there is, then, an accumulation of money, a raking together of a portion of the surplus-value in the form of latent money-capital, which is not intended to function as additional active capital until later, when it swells to a certain volume. That is how the matter looks from the standpoint of the individual capitalist. But simultaneously with the development of capitalist production the credit system also develops. The money-capital which the capitalist cannot as yet employ in his own business is employed by others, who pay him interest for its use. It serves him as money-capital in its specific meaning, as a kind of capital distinguished from productive capital. But it serves as capital in another’s hands. It is plain that with the more frequent realisation of surplus-value and the rising scale on which it is produced, there is an increase in the proportion of new money-capital or money as capital thrown upon the money-market and then absorbed — at least the greater part of it — by extended production.” source