Musnad Abu Ya'la al-Mawsili 117:255

Zuhayr narrated to us, Affan narrated to us, Hammad narrated to us, Thabit informed us, from Anas, that he recited this verse: "Indeed, We have granted you al-Kawthar." The Messenger of Allah (ﷺ) 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 a pungent musk, and its pebbles were pearls."