Washington (CNN) -- At his request, President Barack Obama is ending his inaugural oath with: "So help me God."
Those four words are not legally or constitutionally required, unlike other federal oaths that invoke them as standard procedure.
Historians have wrangled over whether George Washington established precedent by adding the phrase on his own during his first Inaugural acceptance, but the Library of Congress website states he did.
Abraham Lincoln was reported to have said it spontaneously in 1861 and other presidents over the years have followed suit.
A Bible is traditionally used in administering the oath.
Obama took the official oath on Sunday at the White House with his left hand on the family Bible of his wife, Michelle.
At Monday's ceremonial swearing-in at the Capitol, he will use Bibles from Lincoln and Martin Luther King Jr.
Obama took the oath with the same Lincoln Bible in 2009 when he made history as the first African-American president.
The Constitution lays out the exact language to be used in the oath: "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."
Four years ago, a California atheist, Michael Newdow, objected and went to federal court to prevent Chief Justice John Roberts from prompting the president-elect to repeat the "so help me God phrase."
Newdow, along with several non-religious groups, argued the words violate the constitutional ban on government "endorsement" of religion.
The high court ultimately rejected the lawsuit two years ago, and no such legal challenges are expected this time.
Lyndon Johnson's 1965 swearing-in marked a change from tradition.
His wife Claudia, known better as Lady Bird, held the Bible, a job previously managed by the high court's clerk.
Spouses have since had the honor.
CNN's Kevin Liptak contributed to this report.