26 U.S.C. § 6689

(a) Civil penalty If the taxpayer fails to notify the Secretary (on or before the date prescribed by regulations for giving such notice) of a foreign tax redetermination, unless it is shown that such failure is due to reasonable cause and not due to willful neglect, there shall be added to the deficiency attributable to such redetermination an amount (not in excess of 25 percent of the deficiency) determined as follows— (1) 5 percent of the deficiency if the failure is for not more than 1 month, with (2) an additional 5 percent of the deficiency for each month (or fraction thereof) during which the failure continues. (b) Foreign tax redetermination defined For purposes of this section, the term “foreign tax redetermination” means any redetermination for which a notice is required under subsection (c) of section 905 or paragraph (2) of section 404A(g).