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).
Letter to three students (October 1967) as translated in "Solzhenitsyn: A Documentary Record" edited by Leopold Labedz, "The Struggle Intensifies", 1970.