Request for a reference

Hi All Could some one please send me a reference of the fact that the category of presheaves on a Heyting algebra $H$ is equivalent to the category of sheaves on the Heyting algebra of down closed subsets of $H$. I believe this is true in general for any partial order so a reference to either would be greatly appreciated. Dale Garraway Dept. of Math, Eastern Washington dgarraway@ewu.edu [For admin and other information see: http://www.mta.ca/~cat-dist/ ]