What iconic line by Martin Luther King, Jr. was improvised?

The Rev. Dr. Martin Luther King, Jr.’s speech at the March on Washington for Jobs and Freedom in 1963 was unusual among great American speeches in that its most famous words — “I have a dream” — were improvised. King read from his prepared text for most of his speech, which relied on the Bible, the Constitution, and the Declaration of Independence As King neared the end, off to one side, Mahalia Jackson shouted: “Tell them about the dream, Martin!” And he was off, delivering some of the most beloved lines in American history, a speech that he never intended to give and that some of the other civil rights leaders believed no one but the marchers would ever remember.

Source: New York Times