Musnad Ahmad 32:56

Muhammad ibn Fudayl narrated to us, from al-Mukhtar ibn Fulful, who said: I heard Anas ibn Malik saying: The Prophet, may Allah bless him and grant him peace, dozed off into a light sleep, then he raised his head smiling. Either he said to them, or they said to him: "Why did you smile?" The Messenger of Allah, may Allah bless him and grant him peace, said: "A Surah has just been revealed to me." Then he recited: "In the name of Allah, the Most Gracious, the Most Merciful. {Indeed, We have granted you al-Kawthar} [al-Kawthar: 1]," until he finished it. He said: "Do you know what al-Kawthar is?" They said: "Allah and His Messenger know best." He said: "It is a river that my Lord, the Almighty and Majestic, has granted me in Paradise. Upon it is much goodness. My nation will come to it on the Day of Resurrection. Its vessels are the number of the stars. A servant among them will be pulled away, and I will say: 'O Lord, he is from my nation.' It will be said to me: 'You do not know what they innovated after you.'"