26 CFR 601.507: Evidence required to substantiate facts alleged by a recognized representative.

The Internal Revenue Service may require a recognized representative to submit all evidence, except that of a supplementary or incidental character, over a declaration (signed under penalty of perjury) that the recognized representative prepared such submission and that the facts contained therein are true. In any case in which a recognized representative is unable or unwilling to declare his/her own knowledge that the facts are true and correct, the Internal Revenue Service may require the taxpayer to make such a declaration under penalty of perjury.