An idle brain is the devil's workshop

Russian translation

Праздный ум - мастерская дъявола.
Праздность приводит к дурным последствиям, толкает на дурные поступки.

Variations

An idle man is the devil's bolster (cushion, couch).
An idle person is the devil's playfellow.

Synonyms

By doing nothing we learn to do ill.
Satan finds some mischief still for idle hands to do.
Idleness is the mother (root) of all evil (sin, vice).

Comparison

Праздность - мать всех пороков.
Лень до добра не доводит.
Труд человека кормти, а лень портит.

Examples

Tell him to be a good boy and study hard. As idle brain is the devil's workshop. (Biggers)

The letters are soon done and the moment I begin reading she goes to sleep, so I am left just sitting there - with an idle mind - and we all know the use the devil has for idleness. (Christie)