Anas b. Malik said:
The Messenger of Allahﷺ dozed for a short while and raised his smiling. He either said to them(people) or they said to him: Messenger of Allah! Why did you laugh? He said: A surah has been revealed to me just now, and then he recited: “In the name of Allah, Most Gracious. Most Merciful. To thee We have granted the fount (of abundance)” up to the end. When he recited, he asked: Do you know what al-kauthar is? They replied: Allah and his Apostle know best. He said: It is a river which my Lord, the Exalted, has promised me( to grant) in Paradise: there is abundance of good and upon it there is a pond which my people will approach on the Day of Resurrection. There are vessels as numerous as stars(in the sky).

حَدَّثَنَا هَنَّادُ بْنُ السَّرِيِّ، حَدَّثَنَا مُحَمَّدُ بْنُ فُضَيْلٍ، عَنِ الْمُخْتَارِ بْنِ فُلْفُلٍ، قَالَ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ، يَقُولُ : أَغْفَى رَسُولُ اللَّهِ ﷺ إِغْفَاءَةً فَرَفَعَ رَأْسَهُ مُتَبَسِّمًا، فَإِمَّا قَالَ لَهُمْ وَإِمَّا قَالُوا لَهُ : يَا رَسُولَ اللَّهِ لِمَ ضَحِكْتَ فَقَالَ : " إِنَّهُ أُنْزِلَتْ عَلَىَّ آنِفًا سُورَةٌ " . فَقَرَأَ بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ {إِنَّا أَعْطَيْنَاكَ الْكَوْثَرَ} حَتَّى خَتَمَهَا فَلَمَّا قَرَأَهَا قَالَ : " هَلْ تَدْرُونَ مَا الْكَوْثَرُ " . قَالُوا : اللَّهُ وَرَسُولُهُ أَعْلَمُ . قَالَ : " فَإِنَّهُ نَهْرٌ وَعَدَنِيهِ رَبِّي عَزَّ وَجَلَّ فِي الْجَنَّةِ، وَعَلَيْهِ خَيْرٌ كَثِيرٌ عَلَيْهِ حَوْضٌ تَرِدُ عَلَيْهِ أُمَّتِي يَوْمَ الْقِيَامَةِ آنِيَتُهُ عَدَدُ الْكَوَاكِبِ " .