The chain, not the balance. Keep the purchase records showing how the property was originally acquired and paid for, the sale contract, the completion or settlement statement, evidence of proceeds arriving in a named account, and the tax reporting of the disposal. Then show the same money leaving that account for the enterprise.
Collect records while they are easy to get
Banks, brokerages and registries respond slowly, and older transactions are harder to reconstruct once accounts close. Order statements and certified copies as soon as a sale is contemplated, not after a request for evidence arrives. Where a mortgage was repaid from the sale, keep the lender's statements too, since a shortfall between price and proceeds otherwise looks unexplained.
Note which documents will need certified translation if any were issued in another language. Order the historical records first, because they are the part nobody can accelerate later. How the property was originally acquired and paid for is often the hardest link in the chain, particularly where the purchase predates current banking systems or where a mortgage has since been discharged.
Institutions retain records for limited periods and respond slowly to requests about closed accounts. Where a document genuinely cannot be obtained, record what was requested, from whom and when, and ask counsel what secondary evidence would address the gap, rather than leaving it to be explained in a narrative.