Hadithcore
Sahih Muslim, 400 b
sahih

Mukhtar b. Fulful reported that he had heard Anas b. Malik say that the Messenger of Allah (ﷺ) dozed off, and the rest of the hadith is the same as transmitted by Mus-hir except for the words that he (the Prophet) said:

It (Kauthar) is a canal which my Lord the Exalted and the Glorious has promised me in Paradise. There is a tank over it, but he made no mention of the tumblers like the number of the stars.

حَدَّثَنَا أَبُو كُرَيْبٍ، مُحَمَّدُ بْنُ الْعَلاَءِ أَخْبَرَنَا ابْنُ فُضَيْلٍ، عَنْ مُخْتَارِ بْنِ فُلْفُلٍ، قَالَ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ، يَقُولُ أَغْفَى رَسُولُ اللَّهِ صلى الله عليه وسلم إِغْفَاءَةً ‏.‏ بِنَحْوِ حَدِيثِ ابْنِ مُسْهِرٍ غَيْرَ أَنَّهُ قَالَ ‏"‏ نَهْرٌ وَعَدَنِيهِ رَبِّي عَزَّ وَجَلَّ فِي الْجَنَّةِ عَلَيْهِ حَوْضٌ ‏"‏ ‏.‏ وَلَمْ يَذْكُرْ ‏"‏ آنِيَتُهُ عَدَدُ النُّجُومِ ‏"‏ ‏.‏

References2 variants
In-Book Reference
Book 4, Hadith 57
USC-MSA web (English) reference
Book 4, Hadith 791 (deprecated numbering scheme)