Hi
Greetings of the day, there are many ways of proving receipts in cash:
1. Issuing receipt of the cash amount received, for that you have to maintain receipt book.
2. Depositing all cash in to Bank A/c so the transaction are co-related by doing so.
3. Maintaining Cash Book on daily basis recording all cash receipts on daily basis.
4. Maintaining bank book.
5. Prepare rent agreement.
6. Application from tenant for his intention to have property on rent.
All such documents have legal standing so AO has no rights question source once you have shown receipts as income and accounted in books.
Thanks!!