Sayyiduna al-Irbad bin Sariyah (may Allah be pleased with him) said: One morning the Messenger of Allah (peace and blessings be upon him) came to us and gave us an admonition from which the hearts became fearful and the eyes shed tears. We said: "O Messenger of Allah, you have admonished us like one bidding farewell, so take a covenant from us." He (peace and blessings be upon him) said: "Hold fast to the fear of Allah, and to listening and obeying, even if he is an Abyssinian slave. For whoever remains after me will see severe disagreement. So hold fast to my Sunnah and the Sunnah of the rightly guided caliphs; bite onto it with your molar teeth. And beware of newly invented matters, for every newly invented matter is misguidance."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15713