Musannaf Ibn Abi Shayba 30:152

Narrated to us Abd al-Wahhab al-Thaqafi, from Humayd, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) 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 the mud, and it was pungent musk. I said: 'O Gabriel, what is this?' He said: 'It is the Kawthar which Allah has given you.'"