Well, it looks like I get to keep my 50,000 internetz because NOBODY guessed where the quote came from! For the two of you who actually care:
In Terry Pratchett's Hatful of Sky, shortly after Tiffany casts out the hiver, she and Granny Weatherwax have a blazing argument in the middle of the road. At the climax of the argument, just as Granny makes her point, she stops and says, "Peace be on this place".
If you haven't read the Tiffany Aching series, I highly recommend it!