From Wikiquotes (go down to Misattributed):

The government of the United States is not, in any sense, founded on the Christian religion.

This statement was made by an official representative of the U.S., but is actually a line from the English version of the Treaty of Tripoli of 1796, initially signed by a representative of the US on 4 November 1796 during Washington's presidency, approved by Congress 7 June 1797 and finally signed by President John Adams on 10 June 1797. Article 11 of it reads:

As the government of the United States of America is not in any sense founded on the Christian Religion,— as it has in itself no character or enmity against the laws, religion or tranquility of Musselmen,— and as the said States never have entered into any war or act of hostility against any Mehomitan nation, it is declared by the parties that no pretext arising from religious opinions shall ever produce an interruption of the harmony existing between the two countries.

Joel Barlow, who had served as Washington's chaplain, and was also a good friend of Paine and Jefferson was the representative in charge of the translation.

(1)“The United States is in no sense founded upon the Christian doctrine.”
— George Washington

(The story about Washington praying alone at Valley Forge, during the winter of----, was invented by social reformer "Parson" Weems; Weems also was the source of the fable about Washington and the cherry tree. Enquirer magazine, _____)

(2)Weems' name would probably be forgotten today, had it not been for the tension between the liveliness of his narratives, contrasted with the "...charge of a want of veracity that is brought against all Weems's writings". The cherry-tree anecdote illustrates this point. Another dubious anecdote found in the Weems biography is that of Washington's prayer during the winter at Valley Forge

I hope this helps!!
-mbrisport

George Washington stated this in the Treaty of Tripoli, ratified by the Continental Congress.