Riyad as-Salihin

Abu Sa'id Al-Khudri (May Allah bepleased with him) reported: The Prophet (ﷺ) said, "The best type of Jihad (striving in the way of Allah) is speaking a true word in the presence of a tyrant ruler."

[Abu Dawud and At-Tirmidhi, who categorized it as Hadith Hasan].