One of my (many) guilty pleasures is watching old Sinbad movies. I like them all, but my favorite by far is The Golden Voyage of Sinbad (and not just because of Caroline Munro).
There's a running catchphrase in the movie that in fact ends the film:
"Trust in Allah, but tie up your camel!"
This is an old Arabian proverb, to be sure, but it's hardly a unique sentiment. For example, Oliver Cromwell famously quipped to his men, "Put your trust in God, my boys, and keep your powder dry!" when they were preparing to invade Ireland. And the sentiment doesn't always have a religious angle to it, either. For instance, there is Reagan's simple maxim, "Trust, but verify." Same basic idea, I think.
So here's a challenge: what's the original source? Barring that, what's the oldest appearance of this sentiment?