#28
On the authority of Abu Najih Al-Erbadh bin Sariah, who said :

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