Try Paul Boller and John George, who co-wrote They Never Said It: A Book of Fake Quotes, Misquotes, & Misleading Attributions
It would seem that the quote is one given by David Barton who makes them up.
Fake Quotations: Washington and Governing without God « Fake History
Certainly, Washington never said that exact quote as his God was of the deist variety. He does make nods to religion in general, but he does not seem to favor any including Christianity. Even in the earliest version there is no evidence that Washington actually said something like it.