Narrated 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).”
Sahih Bukhari, Volume 8, Book76, Number 580