The devil makes work for idle hands
Definition 1 of 1
Pro Tip 1/3Idiomatic Origin
Originates from religious writings suggesting idleness leads to sin and trouble. 
He started hanging out with the wrong crowd - the devil makes work for idle hands.
Pro Tip 2/3Idleness Warning
Used as a caution that not keeping busy may result in harmful behavior. 
She took up painting to stay busy; the devil makes work for idle hands after all.
Pro Tip 3/3Negative Connotation
Implies that being idle or unoccupied is a bad thing. 
I should volunteer more because the devil makes work for idle hands.