The messenger of Allah gave us a sermon by which our hearts were filled with fear and tears came to our eyes. We said: “O Messenger of Allah, it is as though this is a farewell sermon, so councel us.” He said: “I councel you to fear Allah and to give absolute obedience even if a slave becomes your leader. Verily he among you who lives [long] will see great controversy, so you must keep to my sunnah and to the sunnah of the rightly-guided Khalifahs – cling to them stubbornly. Beware of newly invented matters, for every invented matter is an innovation and every innovation is a going astray, and every going astray is in Hell-fire.”
Related by Abu Dawud and Al-Tirmithi, who said that it was a fine and true Hadith.