"So help me God" - not uttered until 1881
The oath of office of the President of the United States is required by the Constitution before the President enters office. As specified in Article Two, Section One, Clause Eight:
"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.'"
Presidents today add the phrase "So help me God," though the Constitution does not require them to do so. There is no record of any president using the term until Chester Arthur was sworn in following the death of President James Garfield in September 1881.