Jami` at-Tirmidhi Book 47, Hadith 413
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.”
“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.”
“Umar used to ask me questions in front of the Companions of the Prophet. So Abdur-Rahman bin Awf said to him: ‘Why do you ask him, while we have children like him?’” He said: “Umar said to him: ‘It is because of what you know (about him).’ So he asked him about this Ayah: ‘When there comes the help of Allah and the Conquest.’ I said: “It is only regarding the (end of the) life span of the Messenger of Allah, informing him of it.” Then he recited the Surat until its end. So Umar said to him: “By Allah! I know not about it, but what you know.”