The account of this particular prayer comes from Isaac Potts, a Valley Forge resident who was 26 year old at the time. Isaac Potts was a Quaker. Like many other Quakers he was opposed to war, and therefore a "Loyalist", one who sided with the British during the American Revolutionary War.
The "Diary and Remembrances" of Reverend Nathaniel Randolph Snowden gives the fullest account of Isaac Pott's encounter with Washington praying:
"I was riding with him (Mr. Potts) in Montgomery County, Penn'a near to the Valley Forge, where the army lay during the war of ye Revolution. Mr. Potts was a Senator in our State & a Whig. I told him I was agreeably surprised to find him a friend to his country as the Quakers were mostly Tories.
He said, "It was so and I was a rank Tory once, for I never believed that America c'd proceed against Great Britain whose fleets and armies covered the land and ocean, but something very extraordinary converted me to the Good Faith!"
"What was that," I inquired? "Do you see that woods, & that plain?" It was about a quarter of a mile off from the place we were riding, as it happened. "There," said he, "laid the army of Washington. It was a most distressing time of ye war, and all were for giving up the Ship but that great and good man. In that woods pointing to a close in view, I heard a plaintive sound as, of a man at prayer. I tied my horse to a sapling & went quietly into the woods & to my astonishment I saw the great George Washington on his knees alone, with his sword on one side and his cocked hat on the other. He was at Prayer to the God of the Armies, beseeching to interpose with his Divine aid, as it was ye Crisis, & the cause of the country, of humanity & of the world.
Such a prayer I never heard from the lips of man. I left him alone praying. I went home & told my wife, I saw a sight and heard today what I never saw or heard before, and just related to her what I had seen & heard & observed. We never thought a man c'd be a soldier & a Christian, but if there is one in the world, it is Washington. She also was astonished. We thought it was the cause of God, & America could prevail."