asdflkajfd, that's a hard one. I want to say it's from the second half of either tCBtS or Voyages, but I really can't remember the context. It doesn't feel exactly like any of the bits where the Doctor's teasing Rose about the supposed prudishness of monks, but I'm kind of getting stuck on that. Since there is no bet hedging permitted, I'm going to say... tCBtS.