Narrated Anas ibn Malik: When the Prophet (ﷺ) of Allah (ﷺ) was lifted to the heavens (for travelling) in Paradise, or as he said, a river whose banks were of transparent or hollowed pearls was presented to him. The angel who was with him struck it with his hand and took out musk. Muhammad (ﷺ) then asked the angel who was with him: What is this? He replied: It is al-Kawthar which Allah has given you.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَاصِمُ بْنُ النَّضْرِ، حَدَّثَنَا الْمُعْتَمِرُ، قَالَ سَمِعْتُ أَبِي قَالَ، حَدَّثَنَا قَتَادَةُ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ : لَمَّا عُرِجَ بِنَبِيِّ اللَّهِ صلى الله عليه وسلم فِي الْجَنَّةِ - أَوْ كَمَا قَالَ - عُرِضَ لَهُ نَهْرٌ حَافَتَاهُ الْيَاقُوتُ الْمُجَيَّبُ أَوْ قَالَ الْمُجَوَّفُ، فَضَرَبَ الْمَلَكُ الَّذِي مَعَهُ يَدَهُ فَاسْتَخْرَجَ مِسْكًا فَقَالَ مُحَمَّدٌ صلى الله عليه وسلم لِلْمَلَكِ الَّذِي مَعَهُ : " مَا هَذَا " . قَالَ : هَذَا الْكَوْثَرُ الَّذِي أَعْطَاكَ اللَّهُ عَزَّ وَجَلَّ .
sunnah.com