If you don't know it you should listen to to that link of Gil Scott-Heron reading it, but here are the words if you prefer not to youtube:

    A rat done bit my sister Nell.
      (with Whitey on the Moon)
    Her face and arms began to swell.
      (and Whitey's on the Moon)
    I can't pay no doctor bill.
      (but Whitey's on the Moon)
    Ten years from now I'll be paying still.
      (while Whitey's on the Moon)