In this case this is not any prefix but just a part of the word, because the fact that there's a 'wiesz' part in here (and 'wiedzieć' in the infinitive) is just a coincidence, it seems. Okay, I found a version claiming that it can be explained by "you can say something only after you already know it". Don't know if it's true, anyway today these words aren't really connected.
"Powiedzieć" is a perfective equivalent of "mówić". See here