From an article found on Google. Another article mentions this stroy was first in a newspaper in the 1850's (with obvious discord in the Union brewing).
Here's part of the article:
In the National Tribune, 1880, an article appeared giving an account of the "Vision of Washington" at Valley Forge. The account was told by a gentleman named Anthony Sherman, who supposedly was at Valley Forge during the winter of 1777-78. The story has been published several times.
Some people will say that it is substantiated by the fact that a copy of the account is in the Library of Congress. This argument of authenticity is misleading in and of itself. The Library of Congress has copies of anything published. That does not indicate accuracy of the content.
I am unaware of any eighteenth-century evidence that corroborates this story. The soldier mentioned as having a first-hand account of the "Vision," Anthony Sherman, was a soldier in the Continental Army. However, according to his pension application, written by him, he states that he was at Saratoga under the command of Benedict Arnold at the end of 1777 and only joined the main forces in 1778 in New Jersey just before the Battle of Monmouth.
in-conclusive then....but interesting.