Sayyiduna al-Irbad bin Sariyah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) gave us an admonition from which the eyes shed tears and the hearts became fearful. We said: "O Messenger of Allah, this seems like the admonition of one bidding farewell, so advise us." He (peace and blessings be upon him) said: "I advise you to fear Allah, and to listen and obey, even if (the ruler) is an Abyssinian slave. For whoever among you lives after me will see much disagreement. So hold fast to my Sunnah and the Sunnah of the rightly guided caliphs (al-Khulafa' al-Rashidun) after me; bite onto it with your molar teeth. And beware of newly invented matters, for every innovation (bid'ah) is misguidance."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15708