Muḥammad ibn Fuḍayl narrated to us from al-Mukhtār ibn Fulful, who said, “I heard Anas ibn Mālik say, ‘The Prophet ﷺ dozed off for a moment, then raised his head smiling. Either he said to them, or they said to him, “Why did you smile?” The Messenger of Allah ﷺ said, “A sūrah has just been revealed to me,” and he recited, “In the name of Allah, the Most Gracious, the Most Merciful. Indeed, We have given you al-Kawthar,” to the end of the sūrah. Then he said, “Do you know what al-Kawthar is?” They said, “Allah and His Messenger know best.” He said, “It is a river my Lord, the Mighty and Majestic, has granted me in Paradise, upon which there is much good. My ummah will come to it on the Day of Resurrection; its vessels are as numerous as the stars. A servant of theirs will be snatched away from it and I will say, ‘My Lord, he is one of my ummah,’ and it will be said to me, ‘You do not know what they introduced after you.’”’” Reference: Musnad Aḥmad 89