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)."
Sahih al-Bukhari Book 81, Hadith 179
← Previous Hadith Once the Prophet (ﷺ) went out and offered the funeral prayers for the martyrs of Uhud, and then went to the pulpit and said, "I am a predecessor for you and I am a witness for you: and by Allah, I am looking at my Fount just now, and the keys of the treasures of the earth (or the keys of the earth) have been given to me: and by Allah, I am not afraid that you will worship others besides Allah after me, but I am afraid that you will strive and struggle against each other over these treasures of the world."

Sahih al-Bukhari Book 81, Hadith 178 Next Hadith → Haritha said that he heard the Prophet saying that his Lake-Fount would be as large as the distance between Sana' and Medina. Al- Mustaurid said to Haritha, "Didn't you hear him talking about the vessels?" He said, "No." Al- Mustaurid said, "The vessels are seen in it as (numberless as) the stars."

Sahih al-Bukhari Book 81, Hadith 180