Associated Press
President Barack Obama receives the oath of office from Chief Justice John Roberts as first lady Michelle Obama (L-R) and his daughters Malia and Sasha listen at the ceremonial swearing-in at the U.S. Capitol during the 57th Presidential Inauguration in Washington on Monday.

The presidential oath of office must be recited with precision. But it's OK to add a tag line.

Like many before him, President Barack Obama, during Sunday's official swearing-in and Monday's public ceremony, ended the oath with the phrase: "So help me God."

It's unclear which president first uttered those four words, which are not part of the oath to be taken under the Constitution.

The Blaze offers a good report of the disputed history of who said what and when, beginning with author Washington Irving claiming to have heard George Washington start tradition in his first inauguration.

"The Constitution Center notes that Americans can never know with 100 percent certainty whether Washington did, indeed, insert the statement, as C-SPAN and other media that would have recorded such an occurrence were not yet in operation," wrote Billy Hallowell. "Regardless, there is still a long and robust history surrounding presidential use and invocation of the words, 'So help me God.'”

The center's Constitution Daily and the Library of Congress offer interesting details into the history of presidential inauguration traditions and precedents.

CNN explained the latest conflict over the phrase occured before Obama's first inauguration when California atheist, physician and lawyer Michael Newdow sued in federal court to prevent Supreme Court Chief Justice John Roberts from prompting the president-elect to repeat "so help me God."

Newdow, along with several nonreligious groups, argued the words violate the constitutional ban on government endorsement of religion.

His appeals, which would have blocked its use this year and in 2017, were also rejected.

But the oath of office is only one mention of God in a ceremony that has historically included religious participation and references from prayers and music to a president's inaugural address.

"I challenge you to find any presidential speech that doesn't make a lot of mention of God," constitutional historian R. B. Bernstein told USA Today.

Blogger Ray Soller, who has taken a personal interest in sorting out the history of God talk in presidential inaugurations, took up Bernstein's challenge and found Washington's second inaugural address in 1793 is the sole exception, according to an email exchange between Bernstein and Soller, forwarded to the Deseret News.

Obama didn't become a second exception Monday. By the third paragraph of his speech, Obama spoke of God: "Freedom is a gift from God. It must be secured by his people here on Earth." According to the transcript, the president makes reference to a supreme being at least seven times.