Frances Wright (1829). “Course of popular lectures; with 3 addresses on various public occasions, and a reply to the charges against the French reformers of 1789”, p.25
Third Annual Message to Congress, 7 Dec. 1903. "No one is above the law" appears in the U.S. Supreme Court case Mississippi v. Johnson (1867) (arguments of counsel). Even earlier, "no officer ... is above the law" is found in a Kentucky Supreme Court case, Johnston v. Commonwealth (1809).