Sayyiduna Samurah (may Allah be pleased with him) reports that the Prophet (peace and blessings be upon him) said: "The most excellent jihad is that a word of truth is spoken before a ruler" (the narrator is in doubt; perhaps the words are:) "before an unjust ruler."
Al-Bazzar recorded it; its chain contains Abu Bakr al-Hudhali, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الفتن - أعاذنا الله منها / 12164