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).
Reference: Sahih al-Bukhari 6578 In-book : Book 81, Hadith 166 USC-MSA web (English) : Vol. 8, Book 76, Hadith 580 (deprecated)
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