Unknown server tag 'asp:ScriptManager'

Currently build a new portal using Ajax for asynchronous calls.  Had the following error:
Unknown server tag 'asp:ScriptManager'

This error has caused me a headache over the last few days.  I finally narrowed it down to a missing line in the web.config.

<pages>
  <controls>
    <add tagPrefix="asp" namespace="System.Web.UI" 
            assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
  </controls>
</pages>