Musannaf Ibn Abi Shayba 27:17

Ali bin Mushir narrated to us, from al-Mukhtar, from Anas bin Malik, who said: While the Messenger of Allah (may Allah bless him and grant him peace) was among us, he dozed off, then he raised his head smiling. We said: "What made 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 good. It is a basin to which my nation will come on the Day of Resurrection. Its vessels are [equal to] the number of the stars. A servant among them will be snatched away, and I will say: 'O Lord, he is from my companions.' And He will say: 'No, you do not know what he introduced [as innovations] after you.'"