The Prophet ﷺ said, "Al-Kawthar is a river in Paradise that my Lord has promised me."
حدثنا محمد بن فضيل حدثنا المختار بن فلفل
عن النبي ﷺ قال الكوثر نهر في الجنة وعدنيه ربي عز وجل
Its chain of transmission is sahih. Mukhtar bin Fulful was deemed reliable by Ahmad, Ibn Main, Abu Hatim, Ajli, and Nasai, and others accepted him, and no one criticized him. The hadith was narrated at length with its full details by Tirmidhi 5/449... 3361, in the tafsir of Surah Kawthar, and he said, hasan sahih. It was also narrated by Ibn Majah 2/1450, no. 4334, and by Hakim 3/171, who graded it sahih, and Dhahabi agreed with him. Its basic version appears in Bukhari 8/732, nos. 4964 and 4965, Fath [Bari numbering], in the tafsir of Surah Kawthar, and it was mentioned earlier from the hadith of Abu Hurairah, 6476.