Al-Kawthar 108:2

According to the reports, when the Prophet's son died, al-'As ibn Wa'il said, "Leave him; he is abtar (cut off, without posterity) — when he dies, his name will die with him." Thereupon the surah was revealed: "Surely We have given you al-Kawthar (abundance). So pray to your Lord and sacrifice. It is the one who hates you who is truly cut off." In the hadith narrated from Anas in Muslim, the revelation of the surah is described and al-Kawthar is identified as a river granted to the Prophet in Paradise.

Müslim 400; Vâhidî, Esbâbü'n-Nüzûl