De Ne Rien Ajouter De Son Propre Cru

I am yet to track down the essay in which this appears, but according to Nicholas Zurbrugg's Beckett and Proust:

Perhaps the most important of all Proust's early dictums is his assertion, in his essay 'John Ruskin', that: 'le premier devoir de l'artiste est de ne rien ajouter de son propre cru' (CSB, 111) (the artist's first duty is never to add anything from his own imagination). (p. 42-43)