Hadith: A man, who was set for travel, asked the Prophet (may Allah's peace and blessings be upon him): Which type of Jihad is better? He replied: A word of truth said to a despotic ruler

عن طارق بن شهاب البجلي الأحمسي رضي الله عنه أنّ رجُلًا سأل النبي صلى الله عليه وسلم وقد وضَع رِجله في الغَرْزِ: أَيُّ الجهاد أفضل؟ قال: «كَلِمَةُ حَقًّ عِند سُلطَان جَائِرٍ».
[صحيح] - [رواه النسائي وأحمد]

Tāriq ibn Shihāb al-Bajali al-Ahmasi (may Allah be pleased with him) reported that a man, who was set for travel, asked the Prophet (may Allah's peace and blessings be upon him): ''Which type of Jihad is better?'' He replied: "A word of truth said to a despotic ruler."
Sahih/Authentic. - [An-Nasaa’i]

Explanation

A man who was set for travel asked the Prophet (may Allah's peace and blessings be upon him): Which type of Jihad is much rewarded? The Prophet (may Allah's peace and blessings be upon him) told him that it is to enjoin an oppressive ruler to do good or to forbid him from doing evil. Jihad is not restricted to fighting the disbelievers; rather, it has levels. The type mentioned in this Hadīth is the highest level in terms of reward, because it makes one vulnerable to being killed or imprisoned because of the ruler's injustice, or because it is seldom that someone takes it upon himself to undertake this perilous task.