Sahih al-Bukhari 4964
Narrated Anas:
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.'
حَدَّثَنَا آدَمُ، حَدَّثَنَا شَيْبَانُ، حَدَّثَنَا قَتَادَةُ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ لَمَّا عُرِجَ بِالنَّبِيِّ صلى الله عليه وسلم إِلَى السَّمَاءِ قَالَ ‏
"‏ أَتَيْتُ عَلَى نَهَرٍ حَافَتَاهُ قِبَابُ اللُّؤْلُؤِ مُجَوَّفًا فَقُلْتُ مَا هَذَا يَا جِبْرِيلُ قَالَ هَذَا الْكَوْثَرُ ‏"‏‏.‏