Backup SQL database incorrect syntax

string database = con.Database.ToString();
try
{
    if (textBox1.Text == string.Empty)
    {
        MessageBox.Show("Please Enter Path");
    }
    else
    {
        string cmd = ("BACKUP DATABASE [" + database + "] TO DISK '" + textBox1.Text + "\\" + "Database" +"-"+ DateTime.Now.ToString("yyyy-MM-dd--HHmmss")+".bak'");
        using (SqlCommand command = new SqlCommand(cmd, con))
        {
            if (con.State != ConnectionState.Open)
            {
                con.Open();
            }
            command.ExecuteNonQuery();
            con.Close();
            MessageBox.Show("Backup Completed Successfully");
        }
    }
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}