Sunan Ibn Majah 42

Yahya bin Abu Muta' said:I heard 'Irbad bin Sariyah say: 'One day, the Messenger of Allah (ﷺ) 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 Caliphs, and cling stubbornly to it. And beware of newly-invented matters, for every innovation is a going astray