26 C.F.R. § 55.6065-1
If a return, statement, or other document made under the provisions of Chapter 44 or Subtitle F or the Code or the regulations thereunder with respect to any tax imposed by Chapter 44 of the Code, or the form and instructions issued with respect to such return, statement, or other document, requires that it shall contain or be verified by a written declaration that it is made under the penalties of perjury, it must be so verified by the person or persons required to sign such return, statement, or other document. In addition, any other statement or document submitted under any provision of Chapter 44 or Subtitle F of the Code or regulations thereunder with respect to any tax imposed by Chapter 44 of the Code may be required to contain or be verified by a written declaration that it is made under the penalties of perjury.
26 C.F.R. § 55.6065-1