Sunan an-Nasa'i, Book 39, Hadith 61 (In-Book) ⓘ
It was narrated from Tariq bin Shihab that: a man asked the Prophet, when he had put his leg in the stirrup: "Which kind of Jihad is best?'" He said: " a word of truth spoken before an unjust rulers."
أَخْبَرَنَا إِسْحَاقُ بْنُ مَنْصُورٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ، عَنْ سُفْيَانَ، عَنْ عَلْقَمَةَ بْنِ مَرْثَدٍ، عَنْ طَارِقِ بْنِ شِهَابٍ، أَنَّ رَجُلاً، سَأَلَ النَّبِيَّ صلى الله عليه وسلم وَقَدْ وَضَعَ رِجْلَهُ فِي الْغَرْزِ أَىُّ الْجِهَادِ أَفْضَلُ قَالَ " كَلِمَةُ حَقٍّ عِنْدَ سُلْطَانٍ جَائِرٍ " .
also referenced under: Hadith 4209 (Reference) & Volume 5, Book 39, Hadith 4214 (DarusSalam)