Anas b. Malik said:

The Messenger of Allah ﷺ said: A surah has just been revealed to me. He then recited:”In the name of Allah, the Compassionate, the Merciful. Lo! We have given thee Abundance” until he finished it. Then he asked: Do you know what Abundance (al-Kawthar) is? They replied: Allah and His Apostle know it better. He said: It is a river of which my Lord, the Exalted, the Majestic has promised me to give in Paradise.

حَدَّثَنَا هَنَّادُ بْنُ السَّرِيِّ، حَدَّثَنَا ابْنُ فُضَيْلٍ، عَنِ الْمُخْتَارِ بْنِ فُلْفُلٍ، قَالَ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ، يَقُولُ قَالَ رَسُولُ اللَّهِ ﷺ " أُنْزِلَتْ عَلَىَّ آنِفًا سُورَةٌ " . فَقَرَأَ بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ { إِنَّا أَعْطَيْنَاكَ الْكَوْثَرَ } حَتَّى خَتَمَهَا . قَالَ " هَلْ تَدْرُونَ مَا الْكَوْثَرُ " . قَالُوا اللَّهُ وَرَسُولُهُ أَعْلَمُ . قَالَ " فَإِنَّهُ نَهْرٌ وَعَدَنِيهِ رَبِّي فِي الْجَنَّةِ " .