Musnad Ahmad 27:1212

Husayn ibn Ali narrated to us, from Za'idah, from 'Ata' ibn al-Sa'ib, from Muharib ibn Dithar, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O people, fear injustice, for it is darknesses on the Day of Resurrection."