Anas bin Malik reported,

One day, while the Messenger of Allah ﷺ was among us, he dozed off briefly. Then he raised his head smiling.We asked, "What has made you laugh, Messenger of Allah?"He replied, "A surah has just been revealed to me."Then he recited, "In the name of Allah, the Most Merciful, the Especially Merciful. Indeed, We have granted you al-Kawthar. So pray to your Lord and sacrifice. Indeed, your enemy is the one cut off." [Kawthar 108:1–3].Then he asked, "Do you know what al-Kawthar is?"We said, "Allah and His Messenger know best."He said, "It is a river my Lord has promised me. Upon it is abundant good. It is a pool to which my community will come on the Day of Resurrection, its vessels as numerous as the stars. A servant will be snatched away from among them, and I will say, 'My Lord, he is from my community.' He will reply, 'You do not know what they introduced after you.'"

عن أنس قال

بينا رسول الله ﷺ ذات يوم بين أظهرنا إذ أغفى 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 شانئك 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 أحدث بعدك.