“History is not a web woven with innocent hands. Among all the causes which degrade and demoralize men, power is the most constant and the most active.”

Letter to Edward Newenham (20 October 1792) http://etext.lib.virginia.edu/etcbin/toccer-new2?id=WasFi32.xml&images=images/modeng&data=/texts/english/modeng/parsed&tag=public&part=155&division=div1, these statements and one from a previous letter to Newenham seem to have become combined and altered into a misquotation of Washington's original statements to read:
1790s
Context: Of all the animosities which have existed among mankind, those which are caused by difference of sentiments in religion appear to be the most inveterate and distressing, and ought most to be deprecated. I was in hopes that the enlightened and liberal policy, which has marked the present age, would at least have reconciled Christians of every denomination so far that we should never again see the religious disputes carried to such a pitch as to endanger the peace of society.

Source: Friday (1982), Chapter 24 (p. 244)

1860s, Our Composite Nationality (1869)

Perspectiva communis, translated by, and appearing in the notebooks (C.A._543r) of Leonardo da Vinci, as quoted by Martin Kemp, Leonardo Da Vinci: The Marvellous Works of Nature and Man (2006) p. 112.
Context: Among all the studies of natural causes and reasons, light most delights the contemplators; among the great things of mathematics, the certainty of its demonstrations most illustriously elevates the minds of its investigators; perspective must therefore be preferred to all human discourses and disciplines, in the study in which radiant lines are expounded by means of demonstrations and in which the glory is found not only of mathematics, but also physics: it is adorned with the flowers of one and the other.

Race: A Study in Modern Superstition (1937)
Context: Among the words that can be all things to all men, the word "race" has a fair claim to being the most common, most ambiguous and most explosive. No one today would deny that it is one of the great catchwords about which ink and blood are spilled in reckless quantities. Yet no agreement seems to exist about what race means.

Source: Reflections and Maxims (1746), p. 184.

Misattributed

Epigram wrongly attributed to Thucydides kept in the office of General Powell when he was Chairman of the Joint Chiefs of Staff.
Misattributed