Anas bin Malik reported,

The Prophet ﷺ dozed off briefly, then raised his head smiling. Either he said to them, or they said to him, "Why did you laugh?"The Messenger of Allah ﷺ replied, "A surah was just revealed to me." Then the Messenger of Allah ﷺ recited, "In the name of Allah, the Most Gracious, the Most Merciful. Indeed, We have granted you al-Kawthar," until he completed it.He asked, "Do you know what al-Kawthar is?"They answered, "Allah and His Messenger know best."He said, "It is a river my Lord has granted me in Paradise. Upon it is abundant good. My ummah will come to it on the Day of Resurrection, and its drinking vessels are as numerous as the stars. A servant from among them will be snatched away, and I will say, 'My Lord, he is one of my ummah.' It will be said to me, 'You do not know what they introduced after you.'"

حدثنا محمد بن فضيل عن المختار بن فلفل قال سمعت

أنس بن مالك يقول

أغفى النبي ﷺ إغفاءة فرفع رأسه متبسما إما قال لهم وإما قالوا له لم ضحكت فقال رسول الله ﷺ إنه أنزلت علي آنفا سورة فقرأ رسول الله ﷺ بسم الله الرحمن الرحيم إنا أعطيناك الكوثر حتى ختمها قال هل تدرون ما الكوثر قالوا الله ورسوله أعلم قال هو نهر أعطانيه ربي عز وجل في الجنة عليه خير كثير يرد عليه أمتي يوم القيامة آنيته عدد الكواكب يختلج العبد منهم فأقول يا رب إنه من أمتي فيقال لي إنك لا تدري ما أحدثوا بعدك