The mass of profit cannot increase so long as the same amount of labour is employed, unless the unpaid surplus-labour increases, or, should intensity of exploitation remain the same, unless the number of labourers grows.
 Karl Marx, Das Kapital (1894). copy citation

Context

“The mass of profit remains the same, but it is distributed differently over the total amount of commodities. Nor does this alter the distribution between the labourers and capitalists of the amount of value created by newly added labour. The mass of profit cannot increase so long as the same amount of labour is employed, unless the unpaid surplus-labour increases, or, should intensity of exploitation remain the same, unless the number of labourers grows. Or, both these causes may combine to produce this result. In all these cases — which, however, in accordance with our assumption, presuppose an increase of constant capital as compared to variable, and an increase in the magnitude of total capital — the individual commodity contains a smaller mass of profit and the rate of profit falls even if calculated on the individual commodity.” source