Mary V. Thompson passed this along to me. I hadn't caught it before (neither did she). But apparently, it's not a "new" find, but existed in the record for us to discover all along.
This doesn't, of course, prove Washington was NOT an orthodox Trinitarian Christian, as other orthodox Christians, like George W. Bush, have held the same thing. Though it does reinforce what we've noted about Washington and the other "key Founders" -- that they believed most if not all religions were valid ways to God, that all good men of all religions, even if they are not Jews and Christians, worship the same "Providence." We've seen evidence that Washington, Jefferson and Madison believed the "Great Spirit" that unconverted Natives worship was the same God Jews and Christians worship. Now this is evidence that GW believed Muslims worshipped the same God.
The letter was written on March 31, 1791. The letter was addressed to Yazid ibn-Muhammed, the new Emperor of Morocco, whose father had just passed and Washington sent his condolences as he introduced Thomas Barclay as the new American consul. (Again, thanks to Mary V. Thompson of Mount Vernon for explaining to me the context.)
Here is how Washington closed the letter:
“May that God, whom we both adore, bless your Imperial Majesty with long life, Health and Success, and have you always, great and magnanimous Friend, under his holy keeping.”
Interestingly, Thomas Jefferson wrote these words for GW, but GW signed off on them. Critics may wish to dismiss these as Jefferson's words. But the two times GW ever spoke of Jesus in his extant corpus (one by name, one by example, both in public addresses as opposed to private letters) were written by aides. And in one of the addresses to the unconverted Natives, also written by an aide, GW himself crossed out the word "God" and wrote in "the Great Spirit."
Update: One reason why this quotation may not be more well known is because it is not contained in the official "Fitzpatrick edition." I asked MVT about this and she replied:
I just checked and it doesn’t appear in the Fitzpatrick edition. According to the note in the Papers, Presidential Series, 8:34n, an anonymous individual owns the original signed letter; there is a letterpress copy in the Thomas Jefferson papers at the Library of Congress; and an additional copy in the National Archives.