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

Search 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.