Musnad Ahmad 67:0

Narrated to us by Yahya ibn Sa’id, from Sufyan, who said: Narrated to us by Abu Ishaq, from al-Harith ibn ‘Abd Allah ibn Aws, from his father, who said: I said: "O Messenger of Allah, what do you command me to do?" He said: "I command you to fear Allah, and to listen and obey, even if an Abyssinian slave is appointed over you. And you must follow my Sunnah and the Sunnah of the Rightly-Guided Caliphs after me; hold onto it with your molar teeth. And beware of newly invented matters, for every newly invented matter is an innovation, and every innovation is a misguidance."