“If you happen into company where the talk runs into party, obscemty, scandal, folly, or vice of any kind, you had better pass for morose or unsocial, among people whose good opinion is not worth having, than shock your own conscience by joining in conversation which you must disapprove of.”

Source: The War Hound and the World's Pain (1981), Chapter 10 (p. 112)

The Dignity of Human Nature (1754)

This is from a set of maxims which Washington copied out in his own hand as a school-boy: "Rules of Civility and Decent Behaviour in Company and Conversation http://gwpapers.virginia.edu/documents/the-rules-of-civility/" Rule # 56 written out by Washington ca. 1744:
: These maxims originated in the late sixteenth century in France and were popularly circulated during Washington's time. Washington wrote out a copy of the 110 Rules in his school book when he was about sixteen-years old... During the days before mere hero worship had given place to understanding and comprehension of the fineness of Washington's character, of his powerful influence among men, and of the epoch-making nature of the issues he so largely shaped, it was assumed that Washington himself composed the maxims, or at least that he compiled them. It is a satisfaction to find that his consideration for others, his respect for and deference to those deserving such treatment, his care of his own body and tongue, and even his reverence for his Maker, all were early inculcated in him by precepts which were the common practice in decent society the world over. These very maxims had been in use in France for a century and a half, and in England for a century, before they were set as a task for the schoolboy Washington.
:* Charles Moore in his Introduction to George Washington's Rules of Civility and Decent Behaviour in Company and Conversation (1926) http://gwpapers.virginia.edu/documents/civility/index.html, edited by Charles Moore, xi-xv
Misattributed

Speech to the Labour Party Conference (3 October 1988), quoted in "Scargill in furious attack on reform", The Times (4 October 1988), p. 9

Source: Persuasion

Variant: You own everything that happened to you. Tell your stories. If people wanted you to write warmly about them, they should've behaved better.
Source: Bird by Bird: Some Instructions on Writing and Life

Source: The Restaurant at the End of the Universe

Source: Into the Gauntlet

Source: Austenland