صحيح البخاري

Narrated Ibn `Abbas:

The word 'Al-Kauthar' means the abundant good which Allah gave to him (the Prophet (ﷺ) Muhammad). Abu Bishr said: I said to Sa`id, "Some people claim that it (Al-Kauthar) is a river in Paradise." Sa`id replied, "The river which is in Paradise is one item of that good which Allah has bestowed upon him (Muhammad).