The name 'Request' does not exist in the current context

I used the following code the get the url of privouse page/view. It works well in SharePoint 2010 with reference to the system.web namespace.

prePage = Request.QueryString["source"];
if (prePage == null)
         prePage = "/nbep";

Now, I am working in SharePoint 2013. I got an error "The name 'Request' does not exist in the current context". I know when I add a web part, the namespace is "webpart" vs "user control" in SharePoint 2010. I just wonder if anyone know how to accomplish the need in SharePoint 2013.

Many thanks.