How to stop switching to another tab and keeping it stay at the original tab?

Here is my case, I'm developing application with MVVM pattern.  I have several "docking:DocumentWindow"s  within "docking:TabbedMdiContainer" control. And I bind the property "SelectedIndex" to property "TabChanged"  in my view model. Here is the case, currenly the tab1 is selected, user input something in this tab and then switch to another tab: tab2.  This action will trigger the set method of property "TabChanged" to take some error check for the content user inputs in tab1, if the check result is false, I want to keep the tab1 selected instead of switching to tab2.  But I cannot make it. Whatever I did cannot stop it from switching to tab2.  Is there any way to make it? Any help will do me a big favor, thanks a lot.