Which Latin writer wrote about the assimilation of Greek gods? I need more concretely for Hermes-Mercury. Which Latin writer said (assumed) that Hermes was identified with Mercury? I can find this identification in dictionaries on the web or in books, but I need a Latin quotation.
I'm not sure why you need a quote with these particular constraints, but a trivial search of wikipedia references Interpretatio romana, which is the core of what you want. Assimilation of foreign religions through mapping their gods to Roman gods wasn't something any individual did, it was a social practice. Asking who mapped Mercury to Hermes is like asking who decided that Football would be the US National Sport, or that the Beatles were breakout musicians; these aren't determinations made in advance, they are ex post facto observations. The first person to make the observation doesn't deserve any credit.