United States Constitution
Article. II.
Section. 1.
The executive Power shall be vested in a President of the United States of America. He shall hold his Office during the Term of four Years, and, together with the Vice President, chosen for the same Term, be elected, as follows:
Oath of Office Excerpt:
Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation:–“I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.”
http://www.archives.gov/exhibits/charters/constitution_transcript.html
Student Friendly Version:
Before he begins working as president, he will accept and state the following promise: – “I promise truthfully, in a serious and respectful way, that I will loyally perform the job of the Office of the President of the United States, and will to the best of my ability, keep safe, protect and defend the Constitution of the United States.