How to add Validator on Gridview TemplateField

Question

Hello, 

I have this templatefield in a gridview

 

<asp:TemplateField HeaderText="HeaderText">

              <EditItemTemplate>

                <asp:TextBox ID="textbox1" runat="server" 

                       Text='<%# Bind("Field1") %>'></asp:TextBox>

              </EditItemTemplate>

              <ItemTemplate>

                     <asp:Label ID="Label1" runat="server" Text='<%# Bind("Field1") %>'></asp:Label>

              </ItemTemplate>

     <ItemStyle Width="100px" />

</asp:TemplateField>

 

how can I add a validator to check if it's valid number?

 

Thank you in advance.