e.CommandArgument for asp button is not working

I am developing a asp.net application using C#. I created an .aspx page and placed four buttons on different locations on the page. On server side, I want to use just one click event for all four buttons.

Here is my code:

aspx page

<asp:Button ID="Button1" runat="server" CommandArgument="Button1" onClick = "allbuttons_Click" />
<asp:Button ID="Button2" runat="server" CommandArgument="Button2" onClick = "allbuttons_Click" />
<asp:Button ID="Button3" runat="server" CommandArgument="Button3" onClick = "allbuttons_Click" />
<asp:Button ID="Button4" runat="server" CommandArgument="Button4" onClick = "allbuttons_Click" />

cs page

protected void allbuttons_Click(object sender, EventArgs e)
{
    //Here i want to know which button is pressed
    //e.CommandArgument gives an error
}