Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "On the Day of Resurrection, the most severely punished of people will be an unjust ruler."
Abu Ya'la recorded it, and al-Tabarani recorded it in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat; its chain contains Atiyyah, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الخلافة / 9200