Entrust no man you mistrust.

Is there a ready-made phrase in English?
No ready-made phrase easily comes to mind.
The following did occur to me, however:

Don't let the fox guard the chicken coop.

The more common usage is to say that a situation that exists is "like letting the fox guard the chicken coop", but there's no reason I can see why you couldn't express it as an imperative.

Emotion: smile
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A faithful dog?
Your response strikes me as a bit of a non-sequitur.

Someone or something that isn't going to eat the chickens should guard them!

I'm not conversant with the habits of dogs vis-a-vis chickens. That might be a possibility!

Emotion: smile