Please refer below code.
C#
protected void Stop(object sender, EventArgs e)
{
string message = string.Format("alert('Stopped at {0}:{1}');", sw.Elapsed.Minutes, sw.Elapsed.Seconds);
this.ClientScript.RegisterStartupScript(this.GetType(), "alert", message, true);
sw.Stop();
btnStop.Enabled = false;
btnStart.Enabled = true;
ScriptManager.RegisterStartupScript(sender as Control, this.GetType(), "alert", message, true);
}