Anas bin Malik reported,

The Messenger of Allah ﷺ dozed off briefly, then raised his head smiling. Either he said to them, or they said to him, "Messenger of Allah, why are you smiling?"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" [Kawthar 108:1], until he completed it.When he had finished reciting it, he asked, "Do you know what al-Kawthar is?"They replied, "Allah and His Messenger know best."He said, "It is a river my Lord has promised me in Paradise. It holds abundant good. It has a basin where my community will come to drink on the Day of Resurrection, and its vessels are as numerous as the stars."

حدثنا هناد بن السري حدثنا محمد بن فضيل عن المختار بن فلفل قال سمعت

أنس بن مالك يقول

أغفى رسول الله ﷺ إغفاءة فرفع رأسه متبسما فإما قال لهم وإما قالوا له يا رسول الله لم ضحكت فقال إنه أنزلت علي آنفا سورة فقرأ بسم الله الرحمن الرحيم {إنا أعطيناك الكوثر} حتى ختمها فلما قرأها قال هل تدرون ما الكوثر قالوا الله ورسوله أعلم قال فإنه نهر وعدنيه ربي عز وجل في الجنة وعليه خير كثير عليه حوض ترد عليه أمتي يوم القيامة آنيته عدد الكواكب