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