You do not have to give the buyer a formal statement of the amount of interest. There is no form for providing the information. But you and the buyer have to agree on the amount of interest so that you both report the same amount on your tax returns. It would be best to have the agreement in writing.
At the closing each of you should have been provided an amortization schedule which clearly defines the amount of interest & principal that is paid each month. And you will need to enter THEIR name, SS# and address on your tax return AND they need YOUR name, SS# and address on their tax return ... you must cross reference each other so swap info now so you have it for the returns.