Quotation:  "All tyranny needs to gain a foothold is for people of good conscience to remain silent."

Variations: This quotation has hundreds of variations, which generally are composed of some combination of the following components: 
(All that is necessary/needed/required) OR (all it takes) OR (the best way)) for evil to (triumph OR succeed OR prosper OR prevail OR "win in the world" OR "gain a foothold") is for ("good men" OR "good people" OR "good men and women" OR "people of good conscience") to ("do nothing" OR "say nothing" OR "remain silent")

Sources consulted:

1. Papers of Thomas Jefferson Digital Edition
2. Thomas Jefferson Retirement Papers
3. Thomas Jefferson: Papers collection in Hathi Trust Digital Library

Earliest known appearance in print, attributed to Jefferson: 2005[1]

Status: The variant of this quotation most often attributed to Jefferson is "all tyranny needs to gain a foothold is for people of good conscience to remain silent."

http://tartarus.org/~martin/essays/burkequote.html

http://quoteinvestigator.com/2010/12/04/good-men-do/

Footnotes

• 1. William R. Clark, Petrodollar Warfare: Oil, Iraq and the Future of the Dollar (New Society Publishers, 2005), 22.