Musnad Ahmad 32:1046

Bahz and ‘Affan narrated to us, they said: Hammam narrated to us, Qatadah narrated to us, from Anas, that the Prophet (peace and blessings of Allah be upon him) said: "While I was walking in Paradise, suddenly I was at a river whose banks were domes of hollowed pearl." He said: "I said: 'What is this, O Jibril?' He said: 'This is the Kawthar which your Lord, the Almighty and Majestic, has given you.'" He said: "So I struck it with my hand, and its mud was fragrant musk." And ‘Affan said: "The hollowed."