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 وَعَظَنَا رَسُولُ اللهِ صلی الله عليه و سلم مَوْعِظَةً وَجِلَتْ مِنْهَا الْقُلُوبُ، وَذَرَفَتْ مِنْهَا الْعُيُونُ، فَقُلْنَا: يَا رَسُولَ اللهِ كَأَنَّهَا مَوْعِظَةُ مُوَدِّعٍ، فَأَوْصِنَا، قَالَ: أُوصِيكُمْ بِتَقْوَى اللهِ عَزَّ وَجَلَّ، وَالسَّمْعِ وَالطَّاعَةِ وَإِنْ تَأَمَّرَ عَلَيْكُمْ عَبْدٌ، فَإِنَّهُ مَنْ يَعِشْ مِنْكُمْ فَسَيَرَى اخْتِلاَفاً كَثِيرًا، فَعَلَيْكُمْ بِسُنَّتِي وَسُنَّةِ الْخُلَفَاءِ الرَّاشِدِينَ الْمَهْدِيِّينَ، عَضُّوا عَلَيْهَا بِالنَّوَاجِذِ، وَإِيَّاكُمْ وَمُحْدَثَاتِ الأُمُورِ، فَإِنَّ كُلَّ بِدْعَةٍ ضَلاَلَةٌ

 رَوَاهُ أَبُو دَاوُدَ وَالتِّرْمِذِيُّ وَقَالَ: حَدِيثٌ حَسَنٌ صَحِيحٌ

It was narrated on the authority of Abu Najih al-Irbad bin Sariyah, radiyallahu 'anhu, who said:

The Messenger of Allah, sallallahu 'alayhi wasallam, delivered an admonition that made our hearts fearful and our eyes tearful. We said, "O Messenger of Allah, it is as if this were a farewell sermon, so advise us." He said, "I enjoin you to have Taqwa of Allah and that you listen and obey, even if a slave is made a ruler over you. He among you who lives long enough will see many differences. So for you is to observe my Sunnah and the Sunnah of the rightly-principled and rightly-guided successors, holding on to them with your molar teeth. Beware of newly-introduced matters, for every innovation (bid'ah) is an error."

[Abu Dawud & Al-Tirmidhi, who says it is an authentic hadith]