It could be a niche quote in an art history book, but it could hardly be qualified as a saying.

I asked around since my first comment and not a single person knew about it.

It's so memorable, probably why it stick in my memory: how can you have a canvas without a wall? The wall is the canvas. Yet the wall simultaneously constrains the canvas, thus allowing it to become the canvas, to become worthy of a canvas. This French idiom says so much without saying practically anything.