Source, in full
Dupré, quoted by Albert Wolff, 1880's, Notes upon certain masters of the XIX century, - printed not published MDCCCLXXXVI (1886), The Art Age Press, 400 N.Y. (written after the exhibition 'Cent Chefs-d'Oeuvres: the Choice of the French Private Galleries', Petit, Paris / Baschet, New York, 1883, p. 35
Notes from Wikiquote
Jules Dupré was passing some Summer-weeks at Cayeux-sur-Mer, looking out on the sea from his window. From this day he transformed in a painter of marines; according to Wolff