27 CFR §24.45

The employer identification number (as defined at 26 CFR 301.7701-12) of the taxpayer who has been assigned such a number will be shown on each return filed pursuant to the provisions of this part, including amended returns. Failure of the taxpayer to include the employer identification number on any return filed pursuant to the provisions of this part may result in the assertion and collection of the penalty prescribed in 27 CFR 70.113 of this chapter. (Pub. L. 87-397, 75 Stat. 828, as amended (26 U.S.C. 6109, 6676))