Sahih al-Bukhari 4966
صحيح البخاري#4966SahihNarrated 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 65:488