Article 2, Section 1, Clause 5

No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.

1.Records of the Federal Convention
2.Joseph Story, Commentaries on the Constitution 3:§§ 1472--73, 1833