"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 in this form:
"It is in an especial manner our duty as a people, with devout reverence and affectionate gratitude, to acknowledge our many and great obligations to Almighty God, and to implore Him to continue and confirm the blessings we experienced. It is impossible to rightly govern the world without God and the Bible. It is impossible to account for the creation of the universe, without the agency of a Supreme Being. It is impossible to govern the universe without the aid of a Supreme Being. It is impossible to reason without arriving at a Supreme Being. Religion is as necessary to reason, as reason is to religion. The one cannot exist without the other. A reasoning being would lose his reason, in attempting to account for the great phenomena of nature, had he not a Supreme Being to refer to."
In the above paragraph the italicized portion appears to be entirely bogus, and there is no actual record of such a statement ever having been made by Washington. The first sentence is an almost accurate rendition of one from Washington's official proclamation, being a portion of this segment:
"In such a state of things it is in an especial manner our duty as a people, with devout reverence and affectionate gratitude, to acknowledge our many and great obligations to Almighty God and to implore Him to continue and confirm the blessings we experience.
Deeply penetrated with this sentiment, I, George Washington, President of the United States, do recommend to all religious societies and denominations, and to all persons whomsoever, within the United States to set apart and observe Thursday, the 19th day of February next as a day of public thanksgiving and prayer, and on that day to meet together and render their sincere and hearty thanks to the Great Ruler of Nations for the manifold and signal mercies which distinguish our lot as a nation..."
It is to be noted that there is genuine piety expressed in this statement, but it is not of any sectarian kind, Christian or otherwise.
Source:
http://www.religioustolerance.org/badquotes.htm