Idleness is a necessity for the mind, as much as work. Talent is ruined by writing too much, and rusted by not writing at all.
 
[L’oisiveté est nécessaire aux esprits, aussi bien que le travail. On se ruine l’esprit à trop écrire; on se rouille à n’écrire pas.]

Joseph Joubert (1754-1824) French moralist, philosopher, essayist, poet

Pensées [Thoughts], ch. 23 “Des Qualités de l’Écrivain [Of the Qualities of Writers],” ¶ 53 (1805) (1850 ed.) [tr. Lyttelton (1899), ch. 22, ¶ 20]
    (Source)

(Source (French)). Alternate translations:

The mind must rest as well as work. To write too much ruins it; to leave off writing rusts it.
[tr. Attwell (1896), ¶ 336]

One ruins the mind with too much writing. One rusts it by not writing at all.
[tr. Auster (1983), 1805 entry]