Trace it from its origin to the seller or escrow agent: savings history, sale of an asset, a gift or loan with its own paperwork, then the transfers themselves. Pair that trail with the purchase agreement, the closing statement, escrow instructions and receipts. The point is a continuous, dated chain that a stranger can follow without explanation.
Document the business you are buying, too
Alongside the funding trail, collect what shows the enterprise is real and operating: tax returns, financial statements, the lease or premises arrangement, licences, payroll records, supplier and customer contracts, and an asset schedule. Reconcile the allocation of purchase price across those assets with the agreement. Where the seller's records are informal, note that early, because it affects both the price negotiation and how confidently the operating history can be presented.
Assemble two files rather than one. The funding trail runs from origin to destination in dated steps: savings history, an asset sale, a gift or loan with its own paperwork, then the transfers, the closing statement, escrow instructions, and receipts. The enterprise file shows the business is real and operating: tax returns, financial statements, the premises arrangement, licences, payroll records, supplier and customer contracts, and an asset schedule.
Reconcile the allocation of purchase price across those assets with the agreement. Where the seller's records are informal, note it early, since that affects both the price negotiation and how confidently the operating history can be presented to anyone.