Sahih Bukhari Volume 8, Book 76, Hadith Number 580.

Narated By Ibn ‘Abbas : The word ‘Al-Kauthar’ means the abundant good which Allah gave to him (the Prophet Muhammad). Abu Bishr said: I said to Said, “Some people claim that it (Al-Kauthar) is a river in Paradise.” Said replied, “The river which is in Paradise is one item of that good which Allah has bestowed upon him (Muhammad).”