Willie Lincoln plaque

[RJNorton]

Laurie, here is what it says in Twenty Days.

"Word soon got about that Mr. Lincoln had been back to the tomb twice to have Willie's coffin opened so that he could look at him again. Dr. Charles Brown had embalmed Willie so perfectly that he did really seem to be only sleeping, and Lincoln could not bear to leave him alone in the cold tomb."

There is no footnote or endnote. I have seen similar wording in a few other books, but I don't ever recall a footnote or an endnote about this. I am just curious if anyone has seen one. I just don't know the original source (if one exists). I am sure it's possible Lincoln did this, but I am unaware regarding where the information comes from.