When the Prophet ﷺ was made to ascend to the Heavens, he said (after his return), "I came upon a river the banks of which were made of tents of hollow pearls. I asked Gabriel. What is this (river?) He replied, 'This is the Kauthar.'  

لَمَّا عُرِجَ بِالنَّبِيِّ ﷺ إِلَى السَّمَاءِ قَالَ أَتَيْتُ عَلَى نَهَرٍ حَافَتَاهُ قِبَابُ اللُّؤْلُؤِ مُجَوَّفًا فَقُلْتُ مَا هَذَا يَا جِبْرِيلُ قَالَ هَذَا الْكَوْثَرُ  

I asked ʿAisha 'regarding the verse:--'Verily we have granted you the Kauthar.' She replied, "The Kauthar is a river which has been given to your Prophet on the banks of which there are (tents of) hollow pearls and its utensils are as numberless as the stars."  

سَأَلْتُهَا عَنْ قَوْلِهِ تَعَالَى {إِنَّا أَعْطَيْنَاكَ الْكَوْثَرَ} قَالَتْ نَهَرٌ أُعْطِيَهُ نَبِيُّكُمْ ﷺ شَاطِئَاهُ عَلَيْهِ دُرٌّ مُجَوَّفٌ آنِيَتُهُ كَعَدَدِ النُّجُومِ رَوَاهُ زَكَرِيَّاءُ وَأَبُو الأَحْوَصِ وَمُطَرِّفٌ عَنْ أَبِي إِسْحَاقَ  

Saʿid bin Jubair said that Ibn ʿAbbas said about Al-Kauthar. "That is the good which Allah has bestowed upon His Apostle." I said to Saʿid bin Jubair. "But the people claim that it is a river in Paradise." Saʿid said, "The river in Paradise is part of the good which Allah has bestowed on His Apostle."  

أَنَّهُ قَالَ فِي الْكَوْثَرِ هُوَ الْخَيْرُ الَّذِي أَعْطَاهُ اللَّهُ إِيَّاهُ قَالَ أَبُو بِشْرٍ قُلْتُ لِسَعِيدِ بْنِ جُبَيْرٍ فَإِنَّ النَّاسَ يَزْعُمُونَ أَنَّهُ نَهَرٌ فِي الْجَنَّةِ فَقَالَ سَعِيدٌ النَّهَرُ الَّذِي فِي الْجَنَّةِ مِنَ الْخَيْرِ الَّذِي أَعْطَاهُ اللَّهُ إِيَّاهُ