26 C.F.R. § 20.2016-1
In accordance with the provisions of section 2016, the executor (or any other person) receiving a refund of any State death taxes or foreign death taxes claimed as a credit under section 2011 or section 2014 shall notify the district director of the refund within 30 days of its receipt. The notice shall contain the following information:
If the refund was in connection with foreign death taxes claimed as a credit under section 2014, the notice shall also contain a statement showing the amount of interest, if any, paid by the foreign country on the refund. Finally, the person filing the notice shall furnish the district director such additional information as he may request. Any Federal estate tax found to be due by reason of the refund is payable by the person or persons receiving it, upon notice and demand, even though the refund is received after the expiration of the period of limitations set forth in section 6501 (see section 6501(c)(5)). If the tax found to be due results from a refund of foreign death tax claimed as a credit under section 2014, such tax shall not bear interest for any period before the receipt of the refund, except to the extent that interest was paid by the foreign country on the refund.
26 C.F.R. § 20.2016-1