The sale agreement and closing statement, the corporate records if the practice was incorporated, several years of tax filings and financial statements showing the income that built its value, and bank records tracing the proceeds to the investment.
The practice's history is part of the source
Proceeds from selling a business are treated as lawful only if the business itself can be shown to have been lawfully operated and its value lawfully accumulated. That usually means producing the share register or ownership records, financial statements for several years, personal and corporate tax filings, and the sale documents including any holdback or earn-out arrangement. Where part of the price is deferred, the file must explain how the invested capital was funded in the meantime.
The path then has to be traced: proceeds into the operating or personal account, any transfer between institutions, any currency conversion, and the final transfer to the enterprise or the partnership. Gaps invite questions about whose money it is, so an explanation of anything missing is better than silence.