3 U.S. Code § 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.
Source(June 25, 1948, ch. 644, 62 Stat. 678.)
Presidential Recordings and Materials Preservation Act