<Window x:Class="OneWayToSource.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
 <StackPanel VerticalAlignment="Center">
 <TextBox Text="{Binding BindingSource, Mode=OneWayToSource}" Width="Auto" Height="30" Margin="5"/>
 <TextBox Text="{Binding BindingSource, Mode=OneWayToSource}" Width="Auto" Height="30" Margin="5"/>
 </StackPanel>
 </Grid>
</Window>

public partial class MainWindow : Window
 {
  private ViewModel model;
  public MainWindow()
  {
   InitializeComponent();
   model = new ViewModel();
   model.PropertyChanged += OnVMPropertyChanged;
   DataContext = model;
  }

  private void OnVMPropertyChanged(object sender, PropertyChangedEventArgs e)
  {
   MessageBox.Show("PropertyChanged to '" + model.BindingSource + "'");
  }
 }

using System.ComponentModel;

namespace OneWayToSource
{
 public class ViewModel : INotifyPropertyChanged
 {
  private string source;
  public string BindingSource
  {
   get { return source; }
   set { source = value; InvokePropertyChanged("BindingSource"); }
  }

  public ViewModel()
  {
   source = "foobar";
  }

  #region Implementation of INotifyPropertyChanged

  public event PropertyChangedEventHandler PropertyChanged;

  private void InvokePropertyChanged(string propertyName)
  {
   if(PropertyChanged != null)
    PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
  }

  #endregion
 }
}