In his 1963-speech “I Have a Dream”, Dr. Martin Luther King Jr. states the purpose of the speech between lines 1-3:
In these lines, MLK is giving the audience the reason for their presence in that March of August 28th: the freedom of African-Americans.
In order to support his speech, MLK cites some U.S seminal documents. Some of them are the Emancipation Proclamation in line 5:
Also, in subsequent lines (line 20-22), he cites the U.S. Constitution and the Declaration of Independence.