Sunan Ibn Majah, Chapter: The Book of the Sunnah

"I heard Irbad bin Sariyah say: 'One day, the Messenger of Allah (saw) stood up among us and delivered a deeply moving speech to us that melted our hearts and caused our eyes to overflow with tears. It was said to him: 'O Messenger of Allah, you have delivered a speech of farewell, so enjoin something upon us.' He said: 'I urge you to fear Allah, and to listen and obey, even if (your leader) is an Abyssinian slave. After I am gone, you will see great conflict. I urge you to adhere to my Sunnah and the path of the Rightly-Guided (Khulafah) Caliphs, and cling stubbornly to it. And beware or newly-invented matters, for every innovation is a going astray.'" (Hasan)