Hra exemption allowed (or not) with unstamped rent receipt?

It is generally advised that the rent receipts should have revenue stamp affixed and PAN of the landlord mentioned in the rent receipt. These are valid documents to be produced if IT assessment officer makes query. However if the rent had been paid in CASH, amount of rent being say Rs4000 per month, is it still necessary to affix revenue stamp? (I knew for transaction above Rs5000, revenue stamp is necessary). Moreover PAN of landlord is necessary in case rent amount is high, perhaps more than 15000 per month. These two conditions have been discussed in various websites with different opinions. I am confused. Can experts in this forum clear this doubt? Thanks. (I know IT department is concerned about fake rent receipts produced by some people to claim HRA exemp. but is it also not valid to have receipts without stamp and PAN of landlord in such cases as above? What does the rule say?)