What was the first film to feature a slow motion death scene and what
source states this?

The gory western "The Wild Bunch" did a lot to popularize slow-motion
death scenes. I don't know whether or not it was technically the
first.

Thanks for the help.  Originaly I thought it was Seven Samuri by Akira
Kurosawa, but I informed a movie relating to a boat or river may be
the first scene.  Keep 'em coming.

The one relating to a boat or river may be "The Deserter" (1933)...
  
"Vsevolod Pudovkin, for instance, used slow motion in a suicide scene
in The Deserter, in which a man jumping into a river seems sucked down
by the slowly splashing waves."
  
( http://en.wikipedia.org/wiki/Slow-motion )

This is a tricky question, because before the late 20s, film cameras
and projectors were hand cranked, and so film was a more fluid medium
than we think of it today and slow motion was a commonly used
technique.  We see silent films at a standard speed, but at the time
the cameraman and also the projectionist (often the same person in the
very early days) would vary the speed as he went along.  A 1919
projectionists handbook instructs them to "vary the speed to suit the
subject being projected... as a rule solemn scenes will be mproved if
the machine moves slowly".  Each time a film was shown was a unique
performance, so the notion of "the first" is problematic.  However,
there were printed instructions to projectionists which were sent out
with the films, and so we know that, for instance, DW Griffiths' Home
Sweet Home (1914) should have included slow motion at the end.  Don't
know whether there are any deaths in this film.  Birth of a Nation
(1915) is a good bet because there are battlefield death scenes which
would presumably have been shot in slow motion to intensify the
emotion.  The coming of synchronized sound demanded cameras with
accurate and consistent frame rates, and the common use of slow motion
disappeared from film until the late 60s (Peckinpah etc)