Musnad Ahmad 32:2135

Bahz narrated to us, Hammam narrated to us, he said: Qatadah informed us, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) said: "While I was walking in Paradise, suddenly I was at a river whose banks were domes of hollowed pearls." He said: "I said: 'What is this, O Gabriel?' 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 soil was fragrant musk."