if (!int.TryParse(value.ToString(), out proposedValue))
            {
                return new ValidationResult(false, "'" + value.ToString() + "' ist no positive integer (>=0).");
            }

            if (proposedValue < 0)
            {
                // Something was wrong.
                return new ValidationResult(false, "Value can't be smaller than 0.");
            }
        }

        // Everything OK.
        return new ValidationResult(true, null);
    }
}

<DataGridTextColumn Header="Shitfs" IsReadOnly="False">
   <DataGridTextColumn.Binding>
       <Binding Path="Shifts">
          <Binding.ValidationRules>
             <validationRules:IsPositiveIntegerRule />
          </Binding.ValidationRules>
       </Binding>
   </DataGridTextColumn.Binding>
</DataGridTextColumn>

<Style TargetType="TextBlock">
    <Style.Triggers>
        <Trigger Property="Validation.HasError" Value="true">
            <Setter Property="ToolTip"
               Value="{Binding RelativeSource={x:Static RelativeSource.Self},
                  Path=(Validation.Errors)[0].ErrorContent}"/>
        </Trigger>
    </Style.Triggers>
</Style>

<Style x:Key="CellEditStyle" TargetType="TextBox">
   <Style.Triggers>
       <Trigger Property="Validation.HasError" Value="true">
           <Setter Property="ToolTip"
                   Value="{Binding RelativeSource={RelativeSource Self},
                   Path=(Validation.Errors)[0].ErrorContent}"/>
       </Trigger>
   </Style.Triggers>
</Style>

<Setter Property="ToolTip" Value="{Binding RelativeSource={RelativeSource Self}, Path=(Validation.Errors)[0].ErrorContent}"/>