As far as I know he ever advocated for it, but he understood it as an inevitable reaction.

Off the tip of my head the only specific quote I can think of is his

"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.”

I don't think he ever shied away from pointing out its tragic inevitability if you push a group of people far enough, but his goal was always to avoid it.

...it is as necessary for me to be as vigorous in condemning the conditions which cause persons to feel that they must engage in riotous activities as it is for me to condemn riots. I think America must see that riots do not develop out of thin air. Certain conditions continue to exist in our society which must be condemned as vigorously as we condemn riots. But in the final analysis, a riot is the language of the unheard.

You’re probably thinking of Malcolm X.

He didn't preach violence, but he did have armed members during marches. There was enough implication of force to keep the police in relative check during marches.