Code for backup database, & error comes in execute non query

Could not locate entry in sysdatabases for database 'abcd'. No entry found with that name. Make sure that the name is entered correctly.
BACKUP DATABASE is terminating abnormally.

   string dbname = "abcd";
    SqlConnection sqlcon = new SqlConnection();
    SqlCommand sqlcmd = new SqlCommand();
    SqlDataAdapter da = new SqlDataAdapter();
    DataTable dt = new DataTable();

protected void btn_Click(object sender, EventArgs e)
    {
        //Mentioned Connection string make sure that user id and password sufficient previlages
        sqlcon.ConnectionString= @"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\abcd.mdf ;Integrated Security=True;User Instance=True";



        //Enter destination directory where backup file stored
        string destdir = "D:\\Backup";

        //Check that directory already there otherwise create 
        if (!System.IO.Directory.Exists(destdir))
        {
            System.IO.Directory.CreateDirectory("D:\\Backup");
        }
       // try
        {
            //Open connection
            sqlcon.Open();
            //query to take backup database
           

           sqlcmd = new SqlCommand("backup database " + dbname + " to disk='" + destdir + "\\" + DateTime.Now.ToString("ddMMyyyy_HHmmss") + ".Bak'", sqlcon);
            sqlcmd.ExecuteNonQuery();
            //Close connection
            sqlcon.Close();
            Response.Write("Backup database successfully");
        }
      // catch (Exception ex)
        {
            Response.Write("Error During backup database!");
        }
    }