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

Hi Team,
1. I created Empty sharepoint project as a sandboxed solution using vs 2013 for SP2013.
2. Then i added new visual webpart to the solution.
3. Then added People picker control to the visual webpart like below.

<SharePoint:ClientPeoplePicker ID="ppFirstApprover" runat="server" AllowMultipleEntities="false" PrincipalSource="All" Width="60%" PrincipalAccountType="User" />

4. While building the solution i am getting the below error.

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

Could any let me know how to fix this.
Referred and googled, But not getting the solution to fix this issue.

Regards,
Dhaya