Sayyiduna al-Irbad bin Sariyah (may Allah be pleased with him) said: After the morning prayer the Messenger of Allah (peace and blessings be upon him) gave an eloquent admonition from which the eyes shed tears and the hearts became fearful. One of his (peace and blessings be upon him) Companions said: "O Messenger of Allah, this is the admonition of one bidding farewell." 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 will see much disagreement. So beware of newly invented matters, for they are innovation (bid'ah). Whoever among you reaches that (time) must hold fast to my Sunnah and the Sunnah of the rightly guided caliphs; let him bite onto it with his molar teeth."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15714