Musnad Ahmad 32:68

Ibn Abi ‘Adi narrated to us, from Humayd, 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 whose banks were tents of pearls. I struck with my hand at what the water was flowing in, and it was musk of a pungent scent. I said: 'What is this, O Jibril?' He said: 'This is the Kawthar which Allah has given to you.'"