The type or namespace name 'Portal' does not exist in the namespace 'Microsoft.SharePoint' (are you missing an assembly reference?)

Hi, I created a User Control in a project and when trying to build it I get this error on "UserControl1.ascx.designer.cs" ... The type or namespace name 'Portal' does not exist in the namespace 'Microsoft.SharePoint' (are you missing an assembly reference?). Where can I find the DLL to reference pls? thanks