Brokerage statements showing contributions and their origin, the holdings over time, and the sales that produced cash, supported by tax filings reporting the relevant gains and by records of any transfer between institutions.
Follow the account, not just the balance
A portfolio is a chain of transactions and the evidence should follow it. Statements should show money entering the account and where it came from, since contributions funded by declared income are straightforward and contributions from elsewhere need their own explanation. Holdings and their history establish that growth was market growth. Sale confirmations and settlement records show the realization that produced the cash. Tax filings reporting the gains connect all of it to a declared record, which is the strongest corroboration available. Transfers between institutions should be evidenced on both sides. Where an account is held jointly, the file should explain whose money it is. Statements in another language require certified English translations.
Hypothetical example: Mei is using an inheritance and later investment gains to fund an immigrant-investor petition. Her first review should build a dated chain from the estate documents through receipt, brokerage statements, sale confirmations, bank transfers, and the investment subscription. The estate records settle the original source; account statements settle the path and growth. She should obtain records from institutions before account closures or archival fees make them difficult to retrieve. The common failure is an unexplained transfer between accounts that forces a reviewer to guess whether the money invested is the money documented.