It was narrated that Anas in Malik said:

"One day when he-the Prophet ﷺ- was still among us, he took a nap, then he raised his head, smiling. We said to him: 'Why are you smiling, O Messenger of Allah?' He said: 'Just now this Surah was revealed to me: In the Name of Allah, the Most Gracious, the Most Merciful. Verily, We have granted you (O Muahmmad) Al-Kawthar. Therefore turn in prayer to your Lord and sacrifice (to Him only). For he who hates you, he will be cut off.' Then he said: 'Do you know what Al-Kawthar is?' We said: 'Allah and His Messenger know best.' He said: 'It is a river that my Lord has promised me in Paradise. Its vessels are more than the number of the stars. My Ummah will come to me, then a man among them will be pulled away and I will say: "O Lord, he is one of my Ummah" and He will say to me: 'You do not know what he did after you were gone."

أَخْبَرَنَا عَلِيُّ بْنُ حُجْرٍ، قَالَ حَدَّثَنَا عَلِيُّ بْنُ مُسْهِرٍ، عَنِ الْمُخْتَارِ بْنِ فُلْفُلٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ بَيْنَمَا ذَاتَ يَوْمٍ بَيْنَ أَظْهُرِنَا - يُرِيدُ النَّبِيَّ ﷺ - إِذْ أَغْفَى إِغْفَاءَةً ثُمَّ رَفَعَ رَأْسَهُ مُتَبَسِّمًا فَقُلْنَا لَهُ مَا أَضْحَكَكَ يَا رَسُولَ اللَّهِ قَالَ " نَزَلَتْ عَلَىَّ آنِفًا سُورَةُ بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ { إِنَّا أَعْطَيْنَاكَ الْكَوْثَرَ * فَصَلِّ لِرَبِّكَ وَانْحَرْ * إِنَّ شَانِئَكَ هُوَ الأَبْتَرُ } " . ثُمَّ قَالَ " هَلْ تَدْرُونَ مَا الْكَوْثَرُ " . قُلْنَا اللَّهُ وَرَسُولُهُ أَعْلَمُ . قَالَ " فَإِنَّهُ نَهْرٌ وَعَدَنِيهِ رَبِّي فِي الْجَنَّةِ آنِيَتُهُ أَكْثَرُ مِنْ عَدَدِ الْكَوَاكِبِ تَرِدُهُ عَلَىَّ أُمَّتِي فَيُخْتَلَجُ الْعَبْدُ مِنْهُمْ فَأَقُولُ يَا رَبِّ إِنَّهُ مِنْ أُمَّتِي . فَيَقُولُ لِي إِنَّكَ لاَ تَدْرِي مَا أَحْدَثَ بَعْدَكَ " .