The U.S. Capital has a private chapel for congressmen, one feature of which is a stained glass window called "Washington's Gethsemane". The window depicts the kneeling figure of George Washington at Valley Forge, praying to God for his soldiers and his country. The Reverend Mason Locke Weems, in his famous birography, "Life of George Washington" first published in 1800, gives an account of what happened at Valley Forge"
In the winter of '77, while Washington, with the American army, lay encamped at Velley Forge, a certain good old friend, of the respectable family and with the name of Potts, if I mistake not, had occasion to pass through the woods near headquarters. Treading his way along the venerable grove, suddenly he heard the sound of a human voice, which as he advanced increased on his ear, and at length became like the voice of one speaking much in earnest. As he approached the spot with a cautious step, whom should he behold, but the commander in chief of the American armies on his knees at prayer. Motionless with surprise, Potts continued on the place 'til the general, having ended his devotions, arose and with a countenance of angel serenity, .to headquarters Frind Poos then went home and on entering his parlour called out to his wife, 'Sarah, my dear! All's well! All's well! George Washington will yet prevail!'
'What's the matter, Isaac?' replied she, 'thee seems moved.'
'We;;, if I seemed moved, tis no more than what I am. I have this day seen what I never expected. Thee knows that I always thought the sword and gospel utterly inconsistent and that no man could be a soldier and a Christian at the same time. But George Washington has this day convinced me of my mistake.'
Then he related what he had seen and concluded with this prophetical remark: 'If George Washington be not a man of God, I am greatly deceived and still more shall i be deceived if God do not , through him, work out a great salvation for America.'"
There are numerous other accounts of George Washington at prayer at Valley Forge. In 1832, eighty-year old Devault Beaver claimed to have received the account directly from Isaac Potts. Another account came from Dr James Ross Snowden, whose father knew Potts, and yet another form Benson J Lossing saying that Washington's close associate at Valley Forge was recorded as saying that Washington frequently used the grove for prayer.

Ken was able to travel in person to Valley Forge to research for this painting.
The story above is from "The Grand Design" by E. Douglas Clark.