Musnad Abu Ya'la al-Mawsili 131:5

Abu Bakr narrated to us, Ali bin Mushir narrated to us, from al-Mukhtar bin Fulful, from Anas, who said: While the Messenger of Allah (peace and blessings of Allah be upon him) was among us one day, he dozed off into a light sleep, then he raised his head smiling. I said, "What makes you laugh, O Messenger of Allah?" He said, "A Surah was just revealed to me." Then he recited: "In the name of Allah, the Most Gracious, the Most Merciful. Indeed, We have granted you al-Kawthar. So pray to your Lord and sacrifice. Indeed, your enemy is the one cut off." Then he said, "Do you know what al-Kawthar is?" We said, "Allah and His Messenger know best." He said, "It is a river that my Lord has promised me, upon which is much goodness. It is a basin to which my Ummah will come on the Day of Resurrection. Its vessels are as numerous as the stars of the sky. A servant among them will be pulled away, and I will say, 'O Lord, he is from my Ummah.' Then He will say, 'Indeed, you do not know what they innovated after you.'"