Yeah, if the OP can give a reference I'd be very interested, too. I've searched for the "original" reference to datalog because I wanted to cite it, but I couldn't find anything like that.

I have a sneaking suspicion that "function-free Prolog" is as old as ordinary Prolog, and "datalog", as an idea separate to Prolog and used as a database language, was born in the database community, but like the OP I have no reference to this.