Couldn’t say for sure (although maybe it’s mentioned in one of the rap histories out there), but this Billboard piece from November 1979 by Radcliffe Joe and Nelson George surely must be a contender. (Also: woo-hoo, Billboard now on Google Books!)
First Feature Article on Rap in a Mainstream Music Publication?
RockCritics.com 
Actually, here’s a Billboard rap piece from half a year before that one (which actually refers this):
http://books.google.ca/books?id=iiQEAAAAMBAJ&pg=PT2&dq=%22jive+talking+NY+DJs+Rapping+Away+in+Black+Discos%22&hl=en&ei=_SsvToP6Mea80AGdo5TeAQ&sa=X&oi=book_result&ct=result&resnum=4&ved=0CD0Q6AEwAw#v=onepage&q=%22jive%20talking%20NY%20DJs%20Rapping%20Away%20in%20Black%20Discos%22&f=false
Some unbelievable stuff in that Billboard issue… an article claiming that “rock-disco fusion” is on the rise and mentioning no examples… a full-page ad for the Buzzcocks’ Singles Going Steady… an editorial titled “Rip Offs Without Interruptions” complaining very bitterly about radio stations playing entire albums and accusing them of subtly encouraging listeners to tape them…