On the authority of Abu Najih Al-Erbadh ibn Sariah (may Allah be pleased with him), who said:
The Messenger of Allah (peace and blessings of Allah be upon him) gave us a sermon by which our hearts were filled with fear and tears came to our eyes.
We said: “O Messenger of Allah (pbuh), it is as though this is a farewell sermon, so counsel us.”
He said: “I counsel 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 Rashidite Caliphs – 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.”
It was related by Abu Dawud and at-Tirmidhi, who stated it as a Hasan (fair) and Sahih (sound) Hadith.