Irbad (may Allah be pleased with him) was a man of the tribe of Banu Sulaym. He said: The Messenger of Allah (peace and blessings be upon him) came out to us and admonished the people, encouraged them and warned them, and said what Allah willed, and he said: "Worship Allah and do not associate anything with Him; obey the one whom Allah has made ruler over your affairs; and do not dispute with the one to whom this matter (of rule) has been entrusted, even if he is a black slave. Hold fast to what you know of the Sunnah of your Prophet and of the rightly guided caliphs, and bite onto it with your molar teeth."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15712