Well brace yourself for a disappointment because you won't find one. That specific "quote" is not a quote, it is a "tagline," bumper sticker, etc.
It has it's roots in:
"Trust, and then verify."
~Russian proverb
Reagan got some mileage out of that one:
There is an old Russian proverb: Trust but verify.
~Ronald Reagan
"
Doveryai, no proveryai - trust but verify."
In God we trust, all others we voice verify.
http://www.etext.org/Zines/ASCII/RAH/rah9307.txt
http://www.geekgarb.com/tees/taglines.htm