Musnad Abu Ya'la al-Mawsili 115:35

Hadbana ibn Khalid narrated to us, Hammam narrated to us, from Qatadah, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) said: "While I was walking in Paradise, I was suddenly at a river whose banks were domes of hollowed pearl. I said, 'What is this, O Jibril?' He said, 'This is the Kawthar which your Lord has given you.' Then the angel struck it with his hand, and its soil was musk of a pungent scent."