It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "The chief of the martyrs is Hamzah bin Abd al-Muttalib, and a man who stood up before a tyrannical ruler and enjoined him to do good and forbade him from evil, and then that ruler killed him (in retribution for speaking the truth)." This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.