Sys.Application.add_load(ApplicationLoadHandler)

function ApplicationLoadHandler(sender, args)
{
    if (!Sys.WebForms.PageRequestManager.getInstance().get_isInAsyncPostBack())
    {    
      Sys.WebForms.PageRequestManager.getInstance().add_initializeRequest(InitializeRequest);
    }
}

function InitializeRequest(sender, args)
{ 
    var prm = Sys.WebForms.PageRequestManager.getInstance();
    if (prm.get_isInAsyncPostBack())
    {
        args.set_cancel(true);
    }
}
if(typeof(Sys) !== "undefined") Sys.Application.notifyScriptLoaded();