3 U.S.C. § 20 : US Code - Section 20: Resignation or refusal of office

      The only evidence of a refusal to accept, or of a resignation of
    the office of President or Vice President, shall be an instrument
    in writing, declaring the same, and subscribed by the person
    refusing to accept or resigning, as the case may be, and delivered
    into the office of the Secretary of State.