Anas bin Malik reported,

One day, while the Messenger of Allah ﷺ was sitting among us, he dozed off briefly, then raised his head smiling.We asked, "What has made you laugh, 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 given you Kawthar. So pray to your Lord and sacrifice. Surely it is the one who hates you who is cut off" [Kawthar 108:1–3].Then he asked, "Do you know what Kawthar is?"We replied, "Allah and His Messenger know best."He said, "It is a river that my Lord has promised me, and upon it is abundant good. It is a pool to which my community will come on the Day of Resurrection, and its vessels number as many as the stars. Then one of the servants will be snatched away from among them, and I will say, 'My Lord, he is one of my community.' And He will say, 'You do not know what he did after you were gone.'"

عن أنس قال

بينا رسول الله ﷺ ذات يوم بين أظهرنا إذأغفى إغفاءة، ثم رفع رأسه مبتسما، فقلنا: ما أضحكك يا رسول الله؟ قال: أنزلت علي آنفا سورة. فقرأ: {بسم الله الرحمن الرحيم إنا أعطيناك الكوثر فصل لربك وانحر إن شانك هو الأبتر}. ثم قال: أتدرون ما الكوثر؟ فقلنا: الله ورسوله أعلم. قال: فإنه نهر وعدنيه ربي عز وجل، عليه خير كثير، هو حوض ترد عليه أمتي يوم القيامة، آنيته عدد النجوم، فيختلج العبد منهم فأقول: رب إنه من أمتي. فيقول: ما تدري ما أحدثت بعدك.