Hadith: Hold fast to my Sunnah and the Sunnah of the rightly guided Caliphs after me

عن أبي نَجِيح العِرْبَاض بن سارية رضي الله تعالى عنه قال: وَعَظَنَا رسولُ الله صلى الله عليه وسلم موعظة وَجِلَتْ منها القلوب وَذَرَفَتْ منها العيون، فقلنا: يا رسول الله كأنها موعظة مودِّعٍ فأوصنا، قال «أوصيكم بتقوى الله عز وجل والسمع والطاعة، وإن تأمَّر عليكم عبد، فإنه مَن يَعِشْ منكم فسيرى اختلافًا كثيرًا، فعليكم بسنتي وسنة الخلفاء الراشدين المهديين عَضُّوا عليها بالنواجذ، وإياكم ومحدثات الأمور فإن كل بدعة ضلالة».
[صحيح] - [رواه أبو داود والترمذي]

Abu Najīh al-‘Irbād ibn Sāriyah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) gave us an eloquent admonition which caused the hearts to become afraid and the eyes to shed tears. Someone then said: "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 a slave were placed in charge of you. Any of you who lives after me will see much differing. So, hold fast to my Sunnah and the Sunnah of the rightly guided Caliphs. Bite onto it with your molar teeth. Beware of newly invented matters (in religion), for every newly invented matter is misguidance."
Sahih/Authentic. - [Abu Dawood]

Explanation

Abu Najīh al-‘Irbād ibn Sāriyah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) delivered a sermon to us that caused our hearts to fear and our eyes to shed tears. We said: O Messenger of Allah, it is as if this is admonition by one who will soon leave us - seeing his earnest effort in doing so. Hence, they asked him to give them advice which they will hold onto after his death. This shows how the Companions were keen on doing good. A person's farewell advice was usually profound and touching, for he will not remain amongst them to repeat his advice again; that's why he tries to say profound words for which he will be fondly remembered. The Prophet (may Allah’s peace and blessings be upon him) said: I advise you to fear Allah Almighty and to hear and obey, even if a slave were placed in charge of you. In other words, the lowest of people become your leader. Do not disdain to obey him. Or if a slave seizes authority over you, obey him, to avoid turbulence and strife. Any of you who lives after me will see much differing. This is what actually happened under some kings and rulers. And as we move away from the Prophet's era, the difference increases. So, hold fast to - or adhere to - my Sunnah and the Sunnah of the rightly guided Caliphs, for they will not run counter to my Sunnah. He chose them because they used to act upon the Sunnah and support and favor it. The caliphs intended in the Hadīth are Abu Bakr as-Siddīq, ‘Umar ibn al-Khattāb, ‘Uthmān ibn ‘Affān, and ‘Ali ibn Abi Tālib (may Allah be pleased with them). Bite onto it with your molar teeth. This indicates strict adherence, for when a person wants to seize something forcefully, he takes it with his teeth. Or the intended meaning may be that they should observe the advice regarding patience over hardships like someone who suffers severe pain that he does not want to show and so he grits his teeth. And beware of newly invented matters - i.e., things newly introduced into the religion without having any basis in the Shariah, for every invented matter is misguidance.