Luther Quoted or Misquoted?

Help me out if anyone can. I have heard this quote many times attributed to Martin Luther: "I'd rather be ruled by a competent Turk than an incompetent Christian"? I have never heard anyone cite the source of the quote. I have googled the quote and in every instance the quote is introduced with something like, "Luther once said..." Is this a true quote of Luther's or is it like the disproven but still widely attributed remark that Luther used tunes from the local bars for his hymns? Any Luther scholars out there?