Anas reported:

One day the Messenger of Allah (ﷺ) was sitting amongst us that he dozed off. He then raised his head smilingly. We said: What makes you smile. Messenger of Allah? He said: A Sura has just been revealed to me, and then recited: In the name of Allah, the Compassionate, the Merciful. Verily We have given you Kauthar (fount of abundance). Therefore turn to your Lord for prayer and offer sacrifice, and surely your enemy is cut off (from the good). Then he (the Prophet) said: Do you know what Kauthar is? We said: Allah and His Messenger know best. The Prophet (ﷺ) said: It (Kauthar) is a canal which my Lord, the Exalted and Glorious has promised me, and there is an abundance of good in it. It is a cistern and my people would come to it on the Day of Resurrection, and tumblers there would be equal to the number of stars. A servant would be turned away from (among the people gathered there). Upon this I would say: My Lord, he is one of my people, and He (the Lord) would say: You do not know that he innovated new things (in Islam) after you. Ibn Hujr made this addition in the hadith:" He (the Prophet) was sitting amongst us in the mosque, and He (Allah) said: (You don't know) what he innovated after you"

حَدَّثَنَا عَلِيُّ بْنُ حُجْرٍ السَّعْدِيُّ، حَدَّثَنَا عَلِيُّ بْنُ مُسْهِرٍ، أَخْبَرَنَا الْمُخْتَارُ بْنُ فُلْفُلٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، ح وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، - وَاللَّفْظُ لَهُ - حَدَّثَنَا عَلِيُّ بْنُ مُسْهِرٍ، عَنِ الْمُخْتَارِ، عَنْ أَنَسٍ، قَالَ بَيْنَا رَسُولُ اللَّهِ صلى الله عليه وسلم ذَاتَ يَوْمٍ بَيْنَ أَظْهُرِنَا إِذْ أَغْفَى إِغْفَاءَةً ثُمَّ رَفَعَ رَأْسَهُ مُتَبَسِّمًا فَقُلْنَا مَا أَضْحَكَكَ يَا رَسُولَ اللَّهِ قَالَ ‏"‏ أُنْزِلَتْ عَلَىَّ آنِفًا سُورَةٌ ‏"‏ ‏.‏ فَقَرَأَ ‏"‏ بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ ‏{‏ إِنَّا أَعْطَيْنَاكَ الْكَوْثَرَ * فَصَلِّ لِرَبِّكَ وَانْحَرْ * إِنَّ شَانِئَكَ هُوَ الأَبْتَرُ‏}‏ ‏"‏ ‏.‏ ثُمَّ قَالَ ‏"‏ أَتَدْرُونَ مَا الْكَوْثَرُ ‏"‏ ‏.‏ فَقُلْنَا اللَّهُ وَرَسُولُهُ أَعْلَمُ ‏.‏ قَالَ ‏"‏ فَإِنَّهُ نَهْرٌ وَعَدَنِيهِ رَبِّي عَزَّ وَجَلَّ عَلَيْهِ خَيْرٌ كَثِيرٌ هُوَ حَوْضٌ تَرِدُ عَلَيْهِ أُمَّتِي يَوْمَ الْقِيَامَةِ آنِيَتُهُ عَدَدُ النُّجُومِ فَيُخْتَلَجُ الْعَبْدُ مِنْهُمْ فَأَقُولُ رَبِّ إِنَّهُ مِنْ أُمَّتِي ‏.‏ فَيَقُولُ مَا تَدْرِي مَا أَحْدَثَتْ بَعْدَكَ ‏"‏ ‏.‏ زَادَ ابْنُ حُجْرٍ فِي حَدِيثِهِ بَيْنَ أَظْهُرِنَا فِي الْمَسْجِدِ ‏.‏ وَقَالَ ‏"‏ مَا أَحْدَثَ بَعْدَكَ ‏"‏ ‏.‏