You "prove" the amount paid...by accessing your purchase confirmation documentation from whatever financial provider sold you that bond . Print it out...it should contain that information as a line item.
One thing to note however, is that you cannot list it as accrued interest paid, unless that bond issued an interest payment after you bought the bond...but must be the same year that you bought it.
1) For instance, if you bought the bond in Nov of 2019, but the bond did not issue it's next interest payout until Feb of 2020, then you should not enter your declaration of accrued interest paid until your file your 2020 taxes early in 2021.
2) a Second complication, though it is a software complication, is that IF your 1099-INT reports interest for several categories..like in any combination of boxes 1, 3, and 8 ...all in the same 1099-INT form, you should remove the interest for the bond type you just purchased, and put it on a separate 1099-INT reporting just the $$ associated with that particular bond type and then put the accrued interest declaration on that separate 1099-INT. Example: if the accrued interest was for a treasury bond you bought, and the financial institution put $$ in boxes 1, 3, and 8 for other holdings you have...you would remove all of the box 3 $$ (and box 12 $$ too if any shows), and create a separate 1099-INT from that provider showing only boxes 3 (and 12) and declaring the accrued interest paid for the bond on that 1099-INT form (a followup page to the main form).
IF you don't do that,the software will divide up the accrued interest proportionally among boxes 1, 3, and 8. Yeah, would be nice someday if TTX would allow separate entries for accrued interest...targeted to the proper box.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
