42 CFR §407.17

1. (a)Who is automatically enrolled. An individual is automatically enrolled for SMI if he or she:
   1. (1)Resides in the United States, except in Puerto Rico;
   2. (2)Becomes entitled to hospital insurance under any of the provisions set forth in §§ 406.10 through 406.15 of this chapter; and
   3. (3)Does not decline SMI enrollment.
2. (b)Opportunity to decline automatic enrollment.
   1. (1)SSA will notify an individual that he or she is automatically enrolled under paragraph (a) of this section and grant the individual a specified period (at least 2 months after the month the notice is mailed) to decline enrollment.
   2. (2)The individual may decline enrollment by submitting to SSA or CMS a signed statement that he or she does not wish SMI.
   3. (3)The statement must be submitted before entitlement begins, or if later, within the time limits set in the notice of enrollment.