سنن النسائي

It was narrated that Anas in Malik said: "One day when he-the Prophet (ﷺ)- was still among us, he took a nap, then he raised his head, smiling. We said to him: 'Why are you smiling, O Messenger of Allah?' He said: 'Just now this Surah was revealed to me: In the Name of Allah, the Most Gracious, the Most Merciful. Verily, We have granted you (O Muahmmad) Al-Kawthar. Therefore turn in prayer to your Lord and sacrifice (to Him only). For he who hates you, he will be cut off.' Then he said: 'Do you know what Al-Kawthar is?' We said: 'Allah and His Messenger know best.' He said: 'It is a river that my Lord has promised me in Paradise. Its vessels are more than the number of the stars. My Ummah will come to me, then a man among them will be pulled away and I will say: "O Lord, he is one of my Ummah" and He will say to me: 'You do not know what he did after you were gone."