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.
Anthony Sherman wrote:
You doubtless heard the story of Washington's going to the thicket to pray in secret for aid and comfort from God, the interposition of whose Divine Providence brought us safely through the darkest days of tribulation. One day, I remember it well, when the chilly winds whistled through the leafless trees, though the sky was cloudless and the sun shown brightly, he remained in his quarters nearly all the afternoon alone. When he came out, I noticed that his face was a shade paler than usual. There seemed to be something on his mind of more than ordinary importance. Returning just after dusk, he dispatched an orderly to the quarters who was presently in attendance. After a preliminary conversation of about an hour, Washington, gazing upon his companion with that strange look of dignity which he alone commanded, related the event that occurred that day.