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."
Reagan got some mileage out of that one:
There is an old Russian proverb: Trust but verify.
"Doveryai, no proveryai
- trust but verify."
In God we trust, all others we voice verify.