If $S$ is a finite subset of a commutative monoid, one can use the notation
$\prod_{x \in S} {x}$
for denoting the product of all elements of $S$.
It is rather obvious, that the same notation can also be used if $S$ is a finite commutative subset of a non-commutative monoid.
I am looking for a reference in the literature (algebra textbooks), where the latter case ($S$ is a finite commutative subset of a non-commutative monoid) was considered.