The best jihad?

There was once a man who came to the Prophet (peace and blessings be upon him), he was on his horse and had his hand on his sword. He asked the Prophet (peace and blessings be upon him) "What is the best struggle for God?"
The Prophet (peace and blessings be upon him) said, "The best struggle is to speak the truth to an oppressive ruler."

Where does this come from? Moses (upon him peace) went to Pharaoh and called him to God. This was hard on him as Pharaoh refused. This story is repeated in the Quran, several times.

You the word Jhd means to struggle not to fight, the Arabic word for war is Harb. Jihad means to struggle not to fight.