There is a Narration in Musnad Ahmad, 28/373

17144 – حَدَّثَنَا الضَّحَّاكُ بْنُ مَخْلَدٍ، عَنْ ثَوْرٍ، عَنْ خَالِدِ بْنِ مَعْدَانَ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَمْرٍو السُّلَمِيِّ، عَنْ عِرْبَاضِ بْنِ سَارِيَةَ، قَالَ: صَلَّى لَنَا رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ الْفَجْرَ، ثُمَّ أَقْبَلَ عَلَيْنَا، فَوَعَظَنَا مَوْعِظَةً بَلِيغَةً، ذَرَفَتْ لَهَا الْأَعْيُنُ  ، وَوَجِلَتْ مِنْهَا الْقُلُوبُ، قُلْنَا أَوْ قَالُوا: يَا رَسُولَ اللهِ، كَأَنَّ هَذِهِ مَوْعِظَةُ مُوَدِّعٍ، فَأَوْصِنَا. قَالَ: ” أُوصِيكُمْ بِتَقْوَى اللهِ وَالسَّمْعِ وَالطَّاعَةِ وَإِنْ كَانَ عَبْدًا حَبَشِيًّا، فَإِنَّهُ مَنْ يَعِشْ مِنْكُمْ يَرَى بَعْدِي اخْتِلَافًا كَثِيرًا، فَعَلَيْكُمْ بِسُنَّتِي وَسُنَّةِ الْخُلَفَاءِ الرَّاشِدِينَ الْمَهْدِيِّينَ، وَعَضُّوا عَلَيْهَا بِالنَّوَاجِذِ، وَإِيَّاكُمْ وَمُحْدَثَاتِ الْأُمُورِ، فَإِنَّ كُلَّ مُحْدَثَةٍ بِدْعَةٌ، وَإِنَّ كُلَّ بِدْعَةٍ ضَلَالَةٌ “

‘Abd Allah – my father – al-Dahhak b. Mukhalad – Thawr – Khalid b. Ma’dan – ‘Abd al-Rahman b. ‘Amr al-Sulami – ‘Irbad b. Sariyah:

The Messenger of Allah, peace be upon him, led us in salat al-fajr. Then, he faced us and delivered an eloquent sermon to us, which caused the eyes to shed tears and also caused fear in the hearts. We said or they said, “O Messenger of Allah, this is a farewell sermon. So, give us instructions.” He said, “I instruct you to fear Allah, and to listen and obey your ruler even if he is an Ethiopian slave. For, whoever lives among you shall witness after me several disagreements. Therefore, follow my Sunnah and thesunnah of the khulafa, who are rashidun and mahdiyun. Bite onto it with your molar teeth. And beware of innovated matters. For, verily, every innovation is a bid’ah; and verily, every bid’ah is misguidance.”

Sheikh Shoaib al-Arnawut terms the Narration Authentic

Also, This Narration is present in Sunan Abi Daood, 4/200, Narration 4607; and Sheikh Albany termed it Authentic

Tirmidhi also mentioned this Narration in His Sunan, 5/44; and termed it Hasan Saheeh. 

This riwayah tells us a number of crucial facts:

1. The Prophet, peace be upon him and his family, was aware that disagreements would arise soon after his death, during the lifetimes of his Sahabah.

2. He ordered his Sahabah to follow his Sunnahand the sunnah of the khulafa after him. He used the singular “sunnah,” and not the plural “sunan,” apparently to emphasize that all thesekhulafa would have a single, uniform sunnah. It was not possible for one of them to have asunnah which would be different from thesunnah of another. The sunnah of all thekhulafa, whatsoever their number, would be one and the same in all cases and circumstances.

To further re-emphasize the point, he used the singular pronoun “it” to refer to his Sunnah and the sunnah of these khulafa jointly. This then establishes that the Sunnah of Muhammad and the sunnah of the khulafa are so perfectly identical and uniform that they are in essence one and the same entity.

The Messenger also described the khulafa asrashidun and mahdiyun. What do these terms mean? Shaykh Dr. al-Fawzan explains in His Sharah Aqeeda Wastia, page 165:

الراشد هو من عرف الحق وعمل به، وضده الغاوي، وهو من عرف الحق ولم يعمل به .وقوله : (المهديين) أي: الذين هداهم الله إلى الحق .

The rashid is he who knows the truth and practises it. His opposite is the deviant, and that is he who knows the truth but does not practise it. His statement (mahdiyin) means those whom Allah guided to the truth.

So, these khulafa know and practise the truth, and are guided to it by Allah Himself. When there are disagreements within the Ummah, everyone must follow these khulafa, because they have only one sunnah among themselves; and their sunnah is also uniform with Sunnah of the Prophet.

Secondly, Abu Bakr, ‘Umar and ‘Uthman did NOT have a single, uniform “sunnah.” In fact, each one of them contradicted the other “severely.” This was why Imam Ibn Hazm got confused and mentioned in His book, al-Ahkam fi Usool-al-Ahkam, 6/76:

وأما قوله صلى الله عليه و سلم عليكم بسنتي وسنة الخلفاء الراشدين فقد علمنا أنه صلى الله عليه و سلم لا يأمر بما لا يقدر عليه ووجدنا الخلفاء الراشدين بعده صلى الله عليه و سلم قد اختلفوا اختلافا شديدا

As for his statement, peace be upon him {follow my Sunnah and the sunnah of thekhulafa who are rashidun}, we know already that he, peace be upon him, never commanded the impossible. Yet, we find that the Rightly Guided Caliphs after him, peace be upon him, had severedisagreements among themselves.

This post is taken from Work of Brother Abu Fatima al-Mohammadi. I have just provided references for What He had Quoted.