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. 



topological monoid from symmetric monoidal category
Well, if you are going to reference me somewhere, I can give you something more explicit. The cited Corollary 11.7 is only about topological monoids. However Theorem 4.10 of "$E\_{\infty}$ spaces, group completions, and permutative categories" ( <http://www.math.uchicago.edu/~may/PAPERS/13.pdf> ) has the precise statement requested: "If $(\mathcal{A},\Box,\ast)$ is a strict monoidal category, then $B\mathcal{A}$ is a topological monoid with product $B\Box$." The result goes on to say precisely what holds with respect to commutativity when $\mathcal{A}$ is permutative (= strict symmetric monoidal). 


