Yahya bin Hamzah (may Allah have mercy on him) relates: (The Caliph) al-Mahdi wrote my letter of appointment to me and commanded me to be firm in judgment, and in his letter he wrote (mentioning the chain): My father narrated to me, from his father, from his grandfather:
It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Your Lord, the Blessed and Exalted, has said: 'By My Might and My Majesty, I shall surely take vengeance on the oppressor in this life and in the Hereafter, and I shall surely take vengeance on the one who saw an oppressed person and was able to help him, yet did not help him.'"
These two hadiths are narrated from al-Mahdi only with this chain, and Yahya bin Hamzah (may Allah have mercy on him) is alone in narrating them.
Hadith Referenceالمعجم الأوسط / باب الألف / 36
Hadith Takhrij«أخرجه الطبراني فى «الكبير» برقم: 10652»