Musnad Ahmad 32:1832

'Abdullah ibn Bakr narrated to us, Humaid narrated to us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I entered Paradise, and suddenly I was at a river flowing, its banks being tents of pearl. I struck with my hand into what was flowing in it, and it was pungent musk. I said: 'O Gabriel, what is this?' He said: 'This is the Kawthar which Allah has given you.'"