C# WPF MVVM event binding

Использую

xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"

 <i:Interaction.Triggers>
   <i:EventTrigger EventName="MouseDown">
   <i:InvokeCommandAction Command="{Binding Path=MouseDownEvent}"/>
 /i:EventTrigger>

Как мне получить кнопку которая нажата?

public ICommand MouseDownEvent { get; set; } 
public MainViewModel(MainWindow window)
{
    this.MouseDownEvent = new RelayCommand(o => {
    });
}