How to save a file in a list box into a .txt file?

How to save a file in a list box into a .txt file?

return string.Format("{0} = {1}",   
JaialaiNumber, "₱" + Bet.ToString()); How to save this file on a .txt file?

Here's the code of my program. ): ^_^ .

public partial class frmJaialai : Form
{
    List<Jaialai> source;

    public frmJaialai()
    {
        InitializeComponent();
    }        

    private void frmJaialai_Load(object sender, EventArgs e)
    {
        source = new List<Jaialai>();          
    }

    private void btnAdd_Click(object sender, EventArgs e)
    {
        int bet;
        if (string.IsNullOrEmpty(txtJaialaiNumber.Text) || !Int32.TryParse(txtBet.Text, out bet))
        {
            MessageBox.Show("Must be a required field.", "Entry Error");
            return;
        }
        var existingProduct = source.Where(x => x.JaialaiNumber == txtJaialaiNumber.Text).SingleOrDefault();
        if (existingProduct != null)
        {
            existingProduct.Bet += bet;
        }
        else
            source.Add(new Jaialai { Bet = bet, JaialaiNumber = txtJaialaiNumber.Text });

        lstJaialaiNumbersList.DataSource = null;
        lstJaialaiNumbersList.DataSource = source;

        txtJaialaiNumber.Text = "";      
    }

    public class Jaialai
    {
        public string JaialaiNumber { get; set; }
        public int Bet { get; set; }

        public override string ToString()
        {
            return string.Format("{0} = {1}", 
                JaialaiNumber, "₱" + Bet.ToString());
        }
    }