http://origin.www.gpo.gov/fdsys/pkg/CFR-1997-title26-vol10/xml/CFR-1997-title26-vol10-sec1-1441-5.xml
§ 1.1441-5
Claiming to be a person not subject to withholding.
(a) Individuals. For purposes of chapter 3 of the Code, an individual's written statement that he or she is a citizen or...