If you file by mail you can attach a copy of the letter, though it's questionable if the IRS even reads that stuff.
If you e-file you can't attach that letter. Instead, you print out your income tax return, attach every single piece of supporting documentation to that income tax return, (W-2's, 1099's, that letter, etc., etc.), and put that income tax return in a safe place. IF the IRS ever questions you about the repayment THEN you reply, describing what happened and attaching a copy of that letter as proof. Heck, you could even attach a copy of the check you wrote. That should be the end of it.
Tom Young
EDIT: I wasn't even thinking that you might be trying to report a 2017 transaction on a 2016 income tax return. If that's the case then the above answer pertains to your 2017 tax year filing, not your 2016 tax year filing.
