It's this speech, by Robert F. Kennedy, given at the City Club of Cleveland on April 5, 1968, the day after Martin Luther King was shot and the day Kennedy gave his famous spontaneous speech in Indianapolis.
It's called "The Mindless Menace of Violence." (Here's the back story on the speech, from the memoir of Kennedy speechwriter Ted Sorensen.)
If I were President Obama, I wouldn't try to outdo this speech. I'd quote it heavily.
UPDATE 1/13 Why I won't analyze President Obama's speech.