This is from the instructions for form 8300. "Generally, if you're in a trade or business and receive more than $10,000 in cash in a single transaction or in related transactions, you must file Form 8300".

here is link: https://www.irs.gov/businesses/small-businesses-self-employed/form-8300-and-reporting-cash-payments-...

Also see: https://www.irs.gov/businesses/small-businesses-self-employed/irs-form-8300-reference-guide#typeofpa...