Jami` at-Tirmidhi Book 47, Hadith 412
“While I was traveling through Paradise, a river appeared before me whose banks had tents of pearl. I said to the angel: ‘What is this?’ He said: ‘This is Al-Kauthar, which Allah has granted you.’” He said: “Then he put his hand in the clay, and removed musk from it, then I was raised up to Sidrat Al-Muntaha so I saw a magnificent light at it.”
“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.”
the Messenger of Allah said: “Al-Kauthar is a river in Paradise, whose banks are of gold, and it flows over pearls and corundum. Its dirt is purer than musk, and its water is sweeter than honey and whiter than milk.”