Paper trails of funding overseas account

Let's say that if we transfer money to foreign brokers or bank, how long should we keep the paper trails?

What if in another 10 or 20 years when we sell our holdings or transfers fund back for retirement, LHDN ask for the source of that money?

Of course capital gain is not taxable, but how many years of paperworks should we retain to proof that it is capital gain and not from trading?