From Sahih Bukhari (Hadith 4966):
Narrated Abu Bishr:Sa`id bin Jubair said that Ibn `Abbas said about Al-Kauthar. "That is the good which Allah has bestowed upon His Apostle." I said to Sa`id bin Jubair. "But the people claim that it is a river in Paradise." Sa`id said, "The river in Paradise is part of the good which Allah has bestowed on His Apostle
Reference: Bukhari 4966