A signed gift instrument stating the amount, the date and that no repayment is expected; evidence of how the donor earned or acquired the funds, such as tax records, employment documents, corporate accounts or sale contracts; and bank statements showing the money leaving their account. Translations are needed where originals are not in English.
Keep the donor's evidence in its own bundle
Assemble the relative's records as a self-contained set with its own index, then attach the transfer documents that link it to the investor's account and the enterprise. That structure makes it obvious which facts belong to which person and avoids mixing two financial histories. Ask the donor's own accountant to confirm figures where a business is involved.
If a record is genuinely unavailable, document why and identify what stands in its place. Add one item to the donor's bundle that is easy to obtain now and awkward later: a short written statement, dated and signed, setting out in the donor's own words where the money came from and that the transfer is a gift with no repayment obligation or retained interest. It costs nothing while the donor is engaged and willing, and it is difficult to obtain if health, distance or a change of feeling intervenes.
Keep it with the financial records rather than in correspondence. Hypothetical example: a donor funding a housing subdivision investment signs a two-paragraph statement at the outset alongside the first tranche of records, and when a question arises three years later about the character of the transfer, the answer is already in the file in the donor's own hand.