Abu Umamah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) was asked, at the time the Jamrah was being stoned: "Which jihad is the most excellent?" He (peace and blessings be upon him) said: "Speaking a word of truth before a tyrannical ruler."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20185