找不到RibbonCommand

<ribbon:RibbonButton Command="ApplicationCommands.Copy" />

public class MyCommand : ICommand
{
    public void Execute(object parameter)
    {
        string hello = parameter as string;
        MessageBox.Show(hello, "World");
    }

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public event EventHandler CanExecuteChanged;
}

<DockPanel.Resources>
    <local:MyCommand x:Key="mycmd"/>
</DockPanel.Resources>

<ribbon:RibbonButton Command="{StaticResource mycmd}" CommandParameter="Hello, command" Label="Copy" LargeImageSource="Images/LargeIcon.png"/>