Musnad Ahmad 32:600

'Abd al-Samad narrated to us, Hammad narrated to us, from Thabit, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I was given al-Kawthar, and it is a river flowing like this upon the surface of the earth. Its banks are domes of pearl, and it is not split. I struck its soil with my hand, and it was musk of a pungent scent, and its pebbles were pearls."