Musnad Ahmad 27:1381

Ali ibn Asim narrated to us, from Ata’—meaning ibn al-Sa’ib—from Muharib—meaning ibn Dithar—from Abdullah ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O people, beware of injustice, for injustice will be darknesses on the Day of Resurrection."