How to set Back Button Icon in NavigationPage?

Hi,

So I have have created a MastetDetailPage with success and a menu Icon set like this MasterDetailPage.Master.Icon="myMenu.png"

My questions is now, when i Push a new Page onto the Detail Page and I want my own "back.png" how do I set it?
So far I had no success doing it!

My projects are both on AppStore and Google Play: Halal Check and SpotOffer

Thanks!