“Integrity is doing the right thing, even if nobody is watching.”

Presumably a paraphrase of "A peculiar virtue in wildlife ethics is that the hunter ordinarily has no gallery to applaud or disapprove of his conduct" or of "Hunting for sport is an improvement ..." above.
Unlikely to be by Leopold, who knew that ethics involves not only doing the right thing, but also determining the right thing in the face of competing desirable criteria.
Misattributed

Source: The Summer Guest

As quoted in Teen Ink : What Matters (2003) by Stephanie H. Meyer, John Meyer, and Peggy Veljkovic, p. 309

As quoted in Subject: <nowiki>[IP http://www.interesting-people.org/archives/interesting-people/200303/msg00427.html</nowiki> John Gilmore on government trustworthiness and spy gear]

Source: The Miraculous Journey of Edward Tulane

Speech at the Republican National Convention (1996-08-13)

WTF Is…? series, Guise of the Wolf (January 26, 2014)

Lecture, The Anti-Slavery Movement http://books.google.pt/books?id=wN9Dj-_wM0IC&pg=PA33&dq=%22I+would+unite+with+anybody+to+do+right+and+with+nobody+to+do+wrong.%22&redir_esc=y#v=onepage&q=%22I%20would%20unite%20with%20anybody%20to%20do%20right%20and%20with%20nobody%20to%20do%20wrong.%22&f=false (1855)
1850s
Variant: I would unite with anybody to do right; and with nobody to do wrong.