Musnad Ahmad 32:1634

'Affan narrated to us, Hammad narrated to us, Thabit informed us, from Anas ibn Malik, that he recited this verse: "Indeed, We have granted you al-Kawthar" [Al-Kawthar: 1]. He said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I was granted al-Kawthar, and it is a river flowing, not dug by digging. Its banks are domes of pearl. I struck my hand into its soil, and it was pungent musk, and its pebbles were pearls."