Musannaf Ibn Abi Shayba 27:16

Thaqafi 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 flowing, its banks being tents of pearl. I struck my hand into the mud, and it was pungent musk. He said: So I said to Gabriel, 'What is this?' He said, 'The river of Al-Kawthar which Allah, the Mighty and Majestic, has given you.'"