Musnad Ahmad 32:1213

Yunus narrated to us, Shaiban narrated to us, from Qatadah, who said: Anas ibn Malik narrated that the Messenger of Allah (peace and blessings of Allah be upon him) said: "While I was walking in Paradise, a river appeared before me, its banks being domes of hollowed pearl. I said: 'What is this, O Gabriel?' He said: 'This is the Kawthar which your Lord has given you.' He said: Then the angel reached out with his hand and brought forth from its mud musk that was pungent."