The phrase itself, from what I can tell, seems to be derived from two sources -- the first being an interview with Mike Wallace in 1966:
MIKE WALLACE: There's an increasingly vocal minority who disagree totally with your tactics, Dr. King.King employed the same phrase on a later occasion, in a speech on "The Other America", given at Grosse Pointe High School - March 14, 1968:
KING: There's no doubt about that. I will agree that there is a group in the Negro community advocating violence now. I happen to feel that this group represents a numerical minority. Surveys have revealed this. The vast majority of Negroes still feel that the best way to deal with the dilemma that we face in this country is through non-violent resistance, and I don't think this vocal group will be able to make a real dent in the Negro community in terms of swaying 22 million Negroes to this particular point of view. And I contend that the cry of "black power" is, at bottom, a reaction to the reluctance of white power to make the kind of changes necessary to make justice a reality for the Negro. I think that we've got to see that a riot is the language of the unheard. And, what is it that America has failed to hear? It has failed to hear that the economic plight of the Negro poor has worsened over the last few years.
Now I wanted to say something about the fact that we have lived over these last two or three summers with agony and we have seen our cities going up in flames. And I would be the first to say that I am still committed to militant, powerful, massive, non-violence as the most potent weapon in grappling with the problem from a direct action point of view. I'm absolutely convinced that a riot merely intensifies the fears of the white community while relieving the guilt. And I feel that we must always work with an effective, powerful weapon and method that brings about tangible results. But it is not enough for me to stand before you tonight and condemn riots. It would be morally irresponsible for me to do that without, at the same time, condemning the contingent, intolerable conditions that exist in our society. These conditions are the things that cause individuals to feel that they have no other alternative than to engage in violent rebellions to get attention. And I must say tonight that a riot is the language of the unheard. And what is it America has failed to hear? It has failed to hear that the plight of the negro poor has worsened over the last twelve or fifteen years. It has failed to hear that the promises of freedom and justice have not been met. And it has failed to hear that large segments of white society are more concerned about tranquility and the status quo than about justice and humanity.Martin Luther King Jr. recognized rioting as a symptom of racial frustration -- but I do not believe he would have ever endorsed such either as a strategy or as a solution.
In the Wallace interview, he also condemned such actions "because riots are self-defeating and socially destructive" -- professing his personal Christian commitment to "militant, powerful, massive, non-violence as the most potent weapon in grappling with the problem from a direct action point of view." We would do well to heed his example.