Sahih Bukhari Volume 8, Book 76, Hadith Number 591.

Narated By Haritha bin Wahb : I heard the Prophet 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).” 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.”