Where is an example of an alliteration in Martin Luther King's speech?
I assume that you are talking about his "I Have A Dream" speech, given at the Lincoln Memorial on August 28th, 1963.
There are multiple examples of alliteration in his "I Have A Dream" speech.
"Now is the time to rise from the dark and desolate valley of segregation..."
"This sweltering summer.."
"The marvelous new militancy..."
"I am not unmindful that some of you have come here out of great trials and tribulations."
"...when all of God's children, black men and white men, Jews and Gentiles,..."
Included below is the entire "I Have A Dream" speech, as well as a copy of the speech with all of the rhetorical techniques used in this speech.