Sahih al-Bukhari 6591
صحيح البخاري#6591SahihNarrated Haritha bin Wahb:I heard the Prophet (ﷺ) mentioning the Lake-Fount (Al-Kauthar), saying, "(The width of the Lake-Fount) is equal to the distance between Medina and Sana' (capital of Yemen)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 81:179