Sahih Bukhari Volume 6, Book 60, Hadith Number 490.

Narrated By Abu Bishr : Said 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.” Said, “The river in Paradise is part of the good which Allah has bestowed on His Apostle.”