Article II, Section 1 of the Constitution

[Note: The 25th Amendment to the Constitution, ratified in 1967, superseded this portion of Article II.]

Article II, Section 1 of the Constitution (1783)

... In Case of the Removal of the President from Office, or of his Death, Resignation, or Inability to discharge the Powers and Duties of the said Office, the Same shall devolve on the Vice President, and the Congress may by Law provide for the Case of Removal, Death, Resignation or Inability, both of the President and Vice President, declaring what Officer shall then act as President, and such Officer shall act accordingly, until the Disability be removed, or a President shall be elected. ...

Source: National Archives