how to arrange the validation in gridview textbox?

<asp:TemplateField HeaderText="Quantity">
      <ItemTemplate>
          <asp:Label ID="lbl_quantity" runat="server" Text='<%# DataBinder.Eval(Container.DataItem,"quantity") %>'>
          </asp:Label>

      </ItemTemplate>

      <EditItemTemplate>
          <asp:TextBox runat="server" ID="txtquantity" Width="90px" onkeypress="return validatenumerics(event);" Text='<%# DataBinder.Eval(Container.DataItem,"quantity") %>'></asp:TextBox>
          <asp:RequiredFieldValidator ID="RFquantity" runat="server" ControlToValidate="txtquntity" Display="None" ErrorMessage="This field is required value" ValidationGroup="quantity">
          </asp:RequiredFieldValidator>
          <ajaxToolkit:ValidatorCalloutExtender ID="VCquantity" runat="Server" TargetControlID="RFquantity"/>

      </EditItemTemplate>
  </asp:TemplateField>