Are you sure? I thought it was Renoir or Batut, or Bresson, or perhaps Watteau, who, when asked for his most useful advice to a new artist, famously uttered this short and mysterious phrase. Could have sworn LaBeouf quoted it in an interview after he collaborated with artist Cantor on their magnum opus.

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.

Coming back at this with a fresh mind, whoever said it could also have meant that every painting should be displayed: it requires a wall to hang on.

As you say, it's not immediately clear what is meant.

Even more evidence of how versatile that French phrase is. There's just so many acceptable meanings to it, and every one of them points to the same conclusion: bounds enable art.