Is there a source for "Whoever says I don't know has given a fatwa"?

I often hear the following quote (or similar quotes): > من قال لا أدري فقد أفتى > > Whoever says I don't know has given a fatwa (opinion) -My own translation- I'd like to know whether this statement has a source? We know that some earlier scholars used to say much more frequently "I don't know" than scholars of these days.