27 CFR 46.102: Employer identification number.

(a) Requirement. The employer identification number (as defined in 26 CFR 301.7701-12) of the taxpayer who has been assigned such a number must be shown on each special tax return, including each amended return, filed under this subpart. Failure of the taxpayer to include the employer identification number may result in the imposition of the penalty specified in § 70.113 of this chapter.

(b) Application for employer identification number. Each taxpayer who files a special tax return and who has not already been assigned an employer identification number must file Internal Revenue Service (IRS) Form SS-4 to apply for one. The taxpayer must apply for and be assigned only one employer identification number, regardless of the number of places of business for which the taxpayer is required to file a special tax return. The taxpayer must apply for the employer identification number no later than 7 days after the filing of the taxpayer's first special (occupational) tax return. IRS Form SS-4 may be obtained from the director of an IRS service center, from any IRS district director, or from https://www.irs.gov.