Source, in full
The Late Show with David Letterman (24 September 2008), while watching a live feed of Katie Couric interviewing with John McCain in the CBS studio; McCain had just canceled his scheduled appearance on Letterman's show that evening, telling him he needed to return to Washington immediately and deal with the economic crisis, quoted in "The David Letterman-John McCain smackdown" by Gary Susman at Popwatch (25 September 2009).