I haven't heard this phrase either. It reminds me of the phrase, "Let sleeping dogs lie," which means that if someone hasn't said anything about a problem, you shouldn't bring it up and remind them.

Thank you for your comments and correction.

Like Adrian, I've never heard that expression. A similar idiom invoking the image of a sleeping bear that I have heard more commonly is "never poke a sleeping bear in the eye." I like this idiom because it is short and to the point. Another similar one that I picked up my from grandfather (so I'm not sure if it is commonly used, or just something that he says) is this: He who puts his hand into a beehive will receive more than a handful of honey. That one made me laugh the first time I heard it.

I have not heard this before, but it is a good expression. A potentially dangerous creature, or person, or people, or country, if quiescent at present, is best not disturbed, as it might react violently if provoked.