Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) said: "On the Day of Resurrection, the leader of all the martyrs will be Sayyiduna Hamzah bin Abd al-Muttalib (may Allah be pleased with him), and a man who stands before an oppressor and enjoins him to good or forbids him from evil, and that ruler has him killed."
Al-Tabarani recorded it in al-Mu'jam al-Awsat, and its chain contains a weak person.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الفتن - أعاذنا الله منها / 12165