Sahih al-Bukhari 81:166
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).
Reference
Sahih al-Bukhari 81:166
In-book reference
Book 81, Hadith 166
Arabic Text
حَدَّثَنِي عَمْرُو بْنُ مُحَمَّدٍ، حَدَّثَنَا هُشَيْمٌ، أَخْبَرَنَا أَبُو بِشْرٍ، وَعَطَاءُ بْنُ السَّائِبِ، عَنْ سَعِيدِ بْنِ جُبَيْرٍ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنه ـ قَالَ الْكَوْثَرُ الْخَيْرُ الْكَثِيرُ الَّذِي أَعْطَاهُ اللَّهُ إِيَّاهُ. قَالَ أَبُو بِشْرٍ قُلْتُ لِسَعِيدٍ إِنَّ أُنَاسًا يَزْعُمُونَ أَنَّهُ نَهَرٌ فِي الْجَنَّةِ. فَقَالَ سَعِيدٌ النَّهَرُ الَّذِي فِي الْجَنَّةِ مِنَ الْخَيْرِ الَّذِي أَعْطَاهُ اللَّهُ إِيَّاهُ.