Given a bifunctor $\otimes: C \times D \to E$ between cocomplete $\infty$-categories which preserves colimits in both variables, it is not so hard to show that for any functors $f:A \to C$ and $g: B \to D$ we have $\mathrm{colim}_{a \in A}
\mathrm{colim}_{b \in B} fa \otimes gb \cong
\mathrm{colim}_{(a,b) \in A \times B} fa \otimes gb$. Is there any reference I can cite, where this is shown rigorously?