Important to recognize that whoever this one person is that is the source for all of the posts I see on the internet attributes the quote to gw's farewell address. Any search of the documents refers to the farewell address documents.
The quote cannot be found by Washington.
"Washington is known to have made some official statements of public piety, but this is not one of them. Though this assertion is very widely reported to have been said in Washington's Farewell Address (17 September 1796), this is not actually the case, as any search of the documents would reveal. It has also been presented as having been part of his Proclamation on 1795-JAN-01 of 1795-FEB-19 as a day of national Thanksgiving"