Hadith: I advise you to fear Allah and to hear and obey, even if an Abyssinian slave. You will see after me great discord. So, hold fast to my Sunnah and the Sunnah of the rightly guided Caliphs

عن العِرْباضِ بن ساريةَ رضي الله عنه قال:
قام فينا رسول الله صلى الله عليه وسلم ذات يوم، فوَعَظَنا مَوعظةً بليغةً وَجِلتْ منها القلوبُ، وذَرَفتْ منها العيونُ، فقيل: يا رسول الله، وعظتَنَا موعظةَ مُودِّعٍ فاعهد إلينا بعهد. فقال: «عليكم بتقوى الله، والسمع والطاعة، وإن عبدًا حبشيًّا، وسترون من بعدي اختلافًا شديدًا، فعليكم بسنتي وسنة الخلفاء الراشدين المهديين، عَضُّوا عليها بالنواجِذ، وإياكم والأمور المحدثات، فإن كل بدعة ضلالة».
[صحيح] - [رواه أبو داود والترمذي وابن ماجه وأحمد] - [سنن ابن ماجه: 42]

Al-‘Irbād ibn Sāriyah (may Allah be pleased with him) reported:
One day, the Messenger of Allah (may Allah's peace and blessings be upon him) gave us an eloquent admonition that caused hearts to become afraid and eyes to shed tears. Someone then said: O Messenger of Allah, this is the admonition of someone who will soon part; so, advise us. He said: "I advise you to fear Allah and to hear and obey, even if an Abyssinian slave. You will see after me great discord. So, hold fast to my Sunnah and the Sunnah of the rightly guided Caliphs. Bite onto it with the molar teeth. Beware of newly invented matters (in religion), for every newly invented matter is misguidance."
Sahih/Authentic. - [Abu Dawood]

Explanation

The Prophet (may Allah's peace and blessings be upon him) delivered an eloquent sermon that filled the Companions' hearts with fear and filled their eyes with tears. They said: O Messenger of Allah, it seems as if it is an admonition by one who will soon leave us - seeing his earnest effort in doing so. Hence, they asked him to give them advice that they would hold onto after him. He said: I advise you to fear Allah Almighty by carrying out duties and avoiding prohibitions. And to hear and obey, i.e., hear and obey the rulers, even if your ruler or the one who seized authority over you is a slave. In other words, if the most inferior among people became your ruler, you should not disdain to obey him, out of fear of turbulence and strife, for whoever lives from you after me will see a great discord. Then, he (may Allah's peace and blessings be upon him) clarified that the way out of this discord is by adhering firmly to his Sunnah and to the Sunnah of the rightly guided Caliphs after him, namely Abu Bakr as-Siddīq, ‘Umar ibn al-Khattāb, ‘Uthmān ibn ‘Affān, and ‘Ali ibn Abi Tālib (may Allah be pleased with them all), and by biting onto it with the molar teeth, i.e., by strictly observing and holding fast to the Sunnah. He (may Allah's peace and blessings be upon him) also warned them against the newly introduced matters in religion, as every religious innovation is a misguidance.