From Sahih Bukhari (Hadith 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
Reference: Bukhari 4964