The Innocents Abroad, by Mark Twain. A travelogue covering the author’s adventures through Europe and the Holy Land.
Page 137 (from the unabridged edition published by Dover in 2003):
We visited the Louvre, at a time when we had no silk purchases in view, and looked at its miles of paintings by the old masters. Some of them were beautiful, but at the same time they carried such evidences about them of the cringing spirit of those great men that we found small pleasure in examining them. Their nauseous adulation of princely patrons was more prominent to me and chained my attention more surely than the charms of color and expression which are claimed to be in the pictures. Gratitude for kindnesses is well, but it seems to me that some of those artists carried it so far that it ceased to be gratitude and become worship. If there is a plausible excuse for the worship of men, then by all means let us forgive Rubens and his brethren.