OK, I am going to guess that there is more to this than handling the Mouse Leave event, but you just need to add
MouseLeave="Button_MouseLeave"/>
to your XAML, and then you create the handler in the code behind like this:-
private void Button_MouseLeave(object sender, MouseEventArgs e)
{
}
I think you should explain more about what you are trying to do inside the mouse leave event so we could help more.