The devil finds work for idle hands to do

Idleness and mischief are linked in [St. Jerome Letters cxxv. xi.] fac et aliquid operis, ut semper te diabolus inveniat occupatum, do something, so that the devil may always find you busy; cf. [c 1386 Chaucer Tale of Melibee l. 1594] Therfore seith Seint Jerome: ‘Dooth somme goode dedes that the devel, which is oure enemy, ne fynde yow nat unocupied.’

In Works of Labour or of Skill I would be busy too: For Satan finds some mischief still for idle Hands to do.
[1715 I. Watts Divine Songs 29]

If the Devil find a Man idle, he'll set him on Work.
[1721 J. Kelly Scottish Proverbs 221]

There is a homely proverb, which speaks a shrewd truth, that whoever the devil finds idle he will employ. ‥What but habitual idleness can hereditary wealth and titles produce?
[1792 M. Wollstonecraft Vindication of Rights of Woman ix.]

Better keep busy, and the devil won't find so much for your idle hands to do.
[1941 A. Updegraff Hills look Down iv.]

There is potential for such a large new intake to become bored. We all know who makes work for idle hands.
[1997 Spectator 1 Nov. 25]

‘I've slaved in your salt mines for twenty years without a day off.’ ‘You can have thirty minutes off.’ ‘Why didn't you give the poor soul more time off?’ ‘Idle hands work for the devil, Poulet.’
[2002 Washington Times 14 Mar. D6 (Crock comic strip)]

Related to: idleness

Bibliography of major proverb collections and works cited from modern editions is available here.