Musnad Abu Ya'la al-Mawsili 129:106

On the authority of Anas, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I entered Paradise, and suddenly I was at a river whose banks were tents of pearls. He said: So I struck my hand into its mud, and it was a pungent musk. I said: 'O Gabriel, what is this?' He said: 'This is the Kawthar which Allah has given you.'"