A continuous chain: tax filings, financial statements, land or equipment sale agreements, and statements for every account the money passed through, plus independent proof for any gift or loan.
Decades of farming income need decades of records
Wealth built gradually from agricultural operations is lawful and entirely ordinary, and it is also harder to document than a single large transaction. The evidence has to show not only that the money exists but how it was earned and where it travelled. Tax filings across the relevant years, financial statements for the operating entities, and agreements for any land, quota or equipment sales form the backbone.
Account statements must cover every step, including accounts used briefly or since closed. Where part of the capital comes from a family member as a gift, that person's own source of funds becomes part of the file. Start this work first; it is the element most likely to determine when the investment can actually proceed.