“it’s raining cats and dogs”, is used to describe an especially heavy rain fall. In other words a downpour (raining very hard).
He who sleeps with dogs will rise up with fleas
He who sleeps with dogs will rise up with fleas is a cautionary statement meaning: If you hang out or associate with the wrong kind of people.
Let sleeping dogs lie
Let sleeping dogs lie is a recommendation to leave things as they are and refrain from discussing a matter any further for fear of instigating trouble.