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.'
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 488
Arabic reference: Book 65, Hadith 4964