Abu 'Abdullah Tariq bin Shihab (May Allah bepleased with him) reported:
A person asked the Prophet (ﷺ) (when he had just put his foot in the stirrup): "What is the highest form of Jihad?" He (ﷺ) said, "Speaking the truth in the presence of a tyrant ruler".[An-Nasa'i].
Arabic/English book reference: Book 1, Hadith 195