Musnad Ahmad 459:2

'Abd al-Rahman ibn Mahdi narrated to us, Mu'awiyah—meaning Ibn Salih—narrated to us, from Damrah ibn Habib, from 'Abd al-Rahman ibn 'Amr al-Sulami, that he heard al-'Irbad ibn Sariyah say: The Messenger of Allah (may Allah bless him and grant him peace) delivered an exhortation to us that caused the eyes to shed tears and the hearts to tremble. We said: "O Messenger of Allah, this is indeed the exhortation of one who is bidding farewell, so what do you entrust to us?" He said: "I have left you upon the clear path, its night is like its day; no one deviates from it after me except one who is doomed. Whoever among you lives will see much disagreement, so you must adhere to what you have known of my Sunnah and the Sunnah of the Rightly-Guided Successors who are rightly-guided. And you must obey, even if it be an Abyssinian slave; bite onto it with your molars. For the believer is like a camel with a nose-ring; wherever it is led, it follows."