Lincoln's Military Guard of Honor

My husband's third great uncle's obituary states he was the last of the military guard of honor at the bier of Abraham Lincoln following Lincoln's assassination.  The uncle was about 20 years old during this time.  I'm having difficulty finding proof of the story.  I've read about the military unit and it was involved, but there's no smoking gun for this uncle and this particular story.  Any ideas on how to prove it?