26 CFR §301.6602-1 — -1 Interest on erroneous refund recoverable by suit.

Any portion of an internal revenue tax (or any interest, assessable penalty, additional amount, or addition to tax) which has been erroneously refunded, and which is recoverable by a civil action pursuant to section 7405, shall bear interest at the annual rate referred to in the regulations under section 6621 from the date of the payment of the refund.