Sahih Bukhari Volume 8, Book 76, Hadith Number 583.

Narated By Anas bin Malik : The Prophet said: “While I was walking in Paradise (on the night of Mi’raj), I saw a river, on the two banks of which there were tents made of hollow pearls. I asked, “What is this, O Gabriel?’ He said, ‘That is the Kauthar which Your Lord has given to you.’ Behold! Its scent or its mud was sharp smelling musk!” (The sub-narrator, Hudba is in doubt as to the correct expression.)