A narrator related: Al-Mahdi wrote me the decree of my appointment and ordered me to be firm in judgements, and in his decree he wrote: My father narrated to me from his father, from his grandfather, from Ibn Abbas (may Allah be pleased with them both), that the Messenger of Allah (peace and blessings be upon him) said: "Your Lord has said: By My might and My majesty, I will certainly take revenge on the oppressor, both in this world and in the Hereafter, and I will certainly take revenge on the one who sees an oppressed person, is able to help him, and yet does not help."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10606