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 hearts became fearful and the eyes shed tears. We said: "O Messenger of Allah, it is as if this is the admonition of 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. Whoever among you lives after me will see much 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; every newly invented matter is misguidance."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15715