US Presidential Trivia: Who was the first President to add "so help me God" to the end of the Presidential Oath?

God and American politics have always been closely entwined.  American politicians have constantly invoked His name as a means of rallying support, garnering attention or playing to their political base.  God is also deeply entrenched in our history - His name, after all, is featured in a variety of US official documents, including the Deceleration of Independence, on our money and our Pledge of Allegiance, among various other items.  So, here is today's question, straight off of our Pocket Trivia: US Presidents game - Who was the first President to add "so help me God" to the end of the Presidential Oath?   Was it Dwight Eisenhower, Abraham Lincoln, George Washington or Harry Truman?

This is a bit of a trick question: George Washington is the correct answer.  If you thought Eisenhower though, don't feel to bad - it was during his Presidency that "under God" was officially added to the Pledge of Allegiance.    Abraham Lincoln has one of my favorite quotes about God and politics:  during the Civil War, when asked if he thought God was on the side of the Union, Lincoln responded, "I am not at all concerned about that.... But it is my constant anxiety and prayer that I and this nation should be on the Lord's side."

Anyway, the official oath of office for the U.S. Presidency reads as follows:  "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."  However, President Washington was believed to have added, "So help me God" to the end of the oath the first time he took it - setting a practice that U.S. Presidents after him would repeat.