Oh, I have. It's just far too long ago, and I had the misfortune of reading a translation. A bad one, at that.
So I couldn't place the quote. I just knew it from different context.
Much the same way it took me years to find a quote that my gradeschool math teacher used and which had lodged itself firmly in my brain. Namely: "To study and not think is a waste. To think and not study is dangerous." - Confucius