Thought is the work of the intellect, reverie is its self-indulgence. To substitute day-dreaming for thought is to confuse a poison with a source of nourishment.

[La pensée est le labeur de l’intelligence, la rêverie en est la volupté. Remplacer la pensée par la rêverie, c’est confondre un poison avec une nourriture.]

Victor Hugo (1802-1885) French writer

Les Misérables, Part 4 “Saint Denis,” Book 2 “Eponine,” ch. 1 (4.2.1) (1862) [tr. Denny (1976)]
    (Source)

(Source (French)). Alternate translations:

Thought is the labour of the intellect, reverie is its pleasure. To replace thought by reverie is to confound poison with nourishment.
[tr. Wilbour (1862)]

Thought is the labor of the intellect, reveries its voluptuousness; substituting reverie for thought is like confounding poison with nutriment.
[tr. Wraxall (1862)]

Thought is the toil of the intelligence, revery its voluptuousness. To replace thought with revery is to confound a poison with a food.
[tr. Hapgood (1887)]

Thought is the labor of the intellect, reverie its pleasure. To replace thought with reverie is to confound poison with nourishment.
[tr. Wilbour/Fahnestock/MacAfee (1987)]

Thought is the exertion of the intellect, daydreaming is its indulgence. To replace thought with daydreaming is to mistake a poison for sustenance.
[tr. Donougher (2013)]