What is the invoice or other documentation required in case reverse charge cases?

Where the tax liability is cast on the recipient, either:
 in case of specified categories of supply of goods or services or both notified by the Government
OR
 where the supplier of goods or services or both is unregistered, the recipient shall issue:
 an invoice in respect of goods or services or both received by him from the supplier who is not registered on the date of receipt of goods or services or both;
 a payment voucher at the time of making payment to the supplier.

(ICAI FAQ PUBLICATION 06-09-2017 -: Tax Invoice, Credit and Debit Notes : FAQ NO. 8)