Trust me, I'm a doctor

I've spent many hours trying to find the origin of the phrase "Trust me, I'm a doctor". I've found no references for it. Does anyone know where it came from?

Cheers,

Persephone

Could it be Zoot (Carol Cleveland) in Monty Python and the Holy Grail? I remember her saying to Galahad (Michael Palin) "Please, we are doctors" as she and her fellow sirens attempted to disrobe and examine the chaste knight.