The Messenger of Allah ﷺ said, "A surah has just been revealed to me." He then 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.He asked, "Do you know what al-Kawthar is?"They replied, "Allah and His Messenger know best."He said, "It is a river that my Lord has promised me in Paradise."
حدثنا هناد بن السري حدثنا ابن فضيل عن المختار بن فلفل قال سمعت
قال رسول الله ﷺ أنزلت علي آنفا سورة فقرأ بسم الله الرحمن الرحيم إنا أعطيناك الكوثر حتى ختمها قال هل تدرون ما الكوثر قالوا الله ورسوله أعلم قال فإنه نهر وعدنيه ربي في الجنة
Takhrij: Muhammad bin Muslim recorded it in the chapter "The Argument of One Who Says the Basmalah Is a Verse at the Beginning of Every Surah Except Baraah," hadith no. 400, from the hadith of Muhammad bin Fudayl through this chain.