MLK is just paraphrasing (or maybe translating) Amos 5:24.

I love Amos - he was a pistol:

4. Hear this, O ye that swallow up the needy, even to make the poor of the land to fail,

5. Saying, When will the new moon be gone, that we may sell corn? and the sabbath, that we may set forth wheat, making the ephah small, and the shekel great, and falsifying the balances by deceit?

6. That we may buy the poor for silver, and the needy for a pair of shoes; yea, and sell the refuse of the wheat?

He also denied that he was a prophet, and described himself as a working man:

Then answered Amos, and said to Amaziah, I was no prophet, neither was I a prophet's son; but I was an herdman, and a gatherer of sycomore fruit:

Personally, I think that the inspiration for that quote may have come from Frederick Douglass in his famous opus on struggle which contains this:

Let me give you a word of the philosophy of reform. The whole history of the progress of human liberty shows that all concessions yet made to her august claims have been born of earnest struggle. The conflict has been exciting, agitating, all-absorbing, and for the time being, putting all other tumults to silence. It must do this or it does nothing. If there is no struggle there is no progress. Those who profess to favor freedom and yet deprecate agitation are men who want crops without plowing up the ground; they want rain without thunder and lightning. They want the ocean without the awful roar of its many waters.

This struggle may be a moral one, or it may be a physical one, and it may be both moral and physical, but it must be a struggle. Power concedes nothing without a demand. It never did and it never will. Find out just what any people will quietly submit to and you have found out the exact measure of injustice and wrong which will be imposed upon them, and these will continue till they are resisted with either words or blows, or with both. The limits of tyrants are prescribed by the endurance of those whom they oppress. In the light of these ideas, Negroes will be hunted at the North and held and flogged at the South so long as they submit to those devilish outrages and make no resistance, either moral or physical. Men may not get all they pay for in this world, but they must certainly pay for all they get. If we ever get free from the oppressions and wrongs heaped upon us, we must pay for their removal. We must do this by labor, by suffering, by sacrifice, and if needs be, by our lives and the lives of others.

with all the voting suppression against Sanders that the revolution will not be peaceful.