Musnad Ahmad 32:211

Yahya narrated to us, from Humayd, from Anas, from the Prophet, peace and blessings of Allah be upon him, who said: "I entered Paradise, and behold, I was at a river whose banks were tents of pearls. I struck my hand into the water's course, and behold, it was pungent musk. I said: 'O Gabriel, what is this?' He said: 'This is the Kawthar which Allah has given you,' or 'which your Lord has given you.'"