R. Kelly Slums It With Cell Phone Music Video

R. Kelly hijacked a random dude's cell phone this weekend in New York -- then recorded a XMAS music video for the guy ... and we got the footage.

It all went down Friday night at Papa Juan Cigar Room in Harlem ... when Kelly showed up with his entourage -- and a bunch of the bar patrons approached him.

One of the guys began recording R.K. with his camera phone -- and that's when Kelly jacked the guy's cell ... and recorded his rendition of "Chestnuts Roasting."