He had stopped working, and nothing is more dangerous than to stop working. It is a habit you lose. A habit easy to give up and difficult to resume.

[Il avait discontinué son travail, et rien n’est plus dangereux que le travail discontinué ; c’est une habitude qui s’en va. Habitude facile à quitter, difficile à reprendre.]

Victor Hugo (1802-1885) French writer

Les Misérables, Part 4 “Saint Denis,” Book 2 “Eponine,” ch. 1 (4.2.1) (1862) [tr. Donougher (2013)]
    (Source)

(Source (French)). Alternate translations:

Nothing is more dangerous than discontinued labour; it is habit lost. A habit easy to abandon, difficult to resume.
[tr. Wilbour (1862); [Wilbour/Fahnestock/MacAfee (1987)]

Nothing is more dangerous than discontinued work, for it is a habit which a man loses -- a habit easy to give up, but difficult to re-acquire.
[tr. Wraxall (1862)]

Nothing is more dangerous than discontinued work; it is a habit which vanishes. A habit which is easy to get rid of, and difficult to take up again.
[tr. Hapgood (1887)]

Nothing is more dangerous than to stop working. It is a habit that can soon be lost, one that is easily neglected and hard to resume.
[tr. Denny (1976)]