Text --> Value

I am trying to bind to a RadMaskedTextInput Value property as shown below, from the a textblock text property, but the binding does not work. Can you tell me what I am doing wrong.

      <telerik:RadMaskedTextInput 
                        Style="{StaticResource RadMaskedTextInputKey}"
                        TextBoxStyle="{StaticResource textboxTrimming}"
                        MaxHeight="40"
                        Mask=""
            Value="abcdefghijklmnopqrstuvwxyz1234567890abcdefghijklmnopqrstuvw">
                <ToolTipService.ToolTip>
                    <TextBlock
                         Text="{Binding RelativeSource ={RelativeSource Mode=FindAncestor,AncestorType={x:Type telerik:RadMaskedTextInput}}, Path=Value}">
                    </TextBlock>                </ToolTipService.ToolTip>
            </telerik:RadMaskedTextInput>