Anas narrated [regarding Allah, Most High's saying] 'Verily We have granted you Al-Kauthar' (108:
1) that the Prophet (ﷺ) said: "It is a river in Paradise." He said: "The Prophet (ﷺ) said: 'I saw a river in Paradise, whose banks had tents were made of pearl. I said: "What is this O Jibril?'" He said: "This is Al-Kauthar which Allah has granted you."
English reference: Vol. 5, Book 44, Hadith 3359
Arabic reference: Book 47, Hadith 3684