Riyad as-Salihin

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, with a sound chain].