Musnad Abu Ya'la al-Mawsili 117:16

Narrated to us Abd al-Rahman ibn Sallam al-Jumahi, narrated to us Hammad, from Thabit and Humayd, from Anas ibn Malik, that the Messenger of Allah (may Allah bless him and grant him peace) said: "I was given the Kawthar, so I struck with my hand into its soil, and behold, it was pungent musk, and its pebbles were pearls, and its two banks were domes of hollowed pearls."