Sayyiduna al-Irbad bin Sariyah (may Allah be pleased with him) narrates that one day after the morning prayer the Messenger of Allah (peace and blessings be upon him) gave them an admonition from which the eyes shed tears and the hearts became fearful. A Companion said: "O Messenger of Allah, this is the admonition of one bidding farewell, so what covenant do you take from us?" He (peace and blessings be upon him) said: "I advise you to fear Allah, and to listen and obey, even if he is an Abyssinian slave. For whoever among you lives after me will see much disagreement. So beware of newly invented matters, for they are misguidance; and hold fast to my Sunnah and the Sunnah of the rightly guided caliphs after me; bite onto it with your molar teeth."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15709