Harry Truman actually said...
Now, Therefore, I, Harry S. Truman, President of the united States of America, under and by virtue of the authority vested in me by the Congress through Public Resolution 767, approved May 3, 1940 (54 Stat. 178), do hereby designate Sunday, May 20, 1951, as 'I Am An American Day', and do set aside that day as a public occasion for the recognition, observance, and commemoration of United States citizenship, and for the special recognition of those of our youth who have attained their majority and of those foreign-born who have become citizens through naturalization during the past year.
03/07/1951