What is the standard reference for the fact that the classifying space of a strict symmetric monoidal category is a topological monoid with respect to the operation induced by the tensor product?

What is the standard reference for the fact that the classifying space of a strict monoidal category is a topological monoid with respect to the operation induced by the tensor product?

EDIT: The first version of the question was stated for strict symmetric monoidal categories, but as was pointed out in the comments, a symmetry is of course not necessary to just get a monoid structure on the classifying space.