Sayyiduna Irbad bin Sariyah (may Allah be pleased with him) said: The Prophet (peace and blessings be upon him) led us in the morning prayer, then turned towards us and gave us an admonition by which hearts trembled and tears flowed. We said: O Messenger of Allah, this is like a farewell admonition, so give us counsel. He (peace and blessings be upon him) said: "Even if your ruler is a slave, obey him, and be mindful of Allah (taqwa). Whoever among you lives on will see much disagreement. Hold fast to my Sunnah and the Sunnah of the rightly guided Caliphs; bite onto it with your molar teeth. Beware of innovations, for every innovation is misguidance."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20338