Musnad Abu Ya'la al-Mawsili 131:7

Abu Khaythama narrated to us, Muhammad ibn Fudayl narrated to us, from al-Mukhtar, from Anas, from the Prophet, may Allah bless him and grant him peace, who said: "Al-Kawthar is a river in Paradise which my Lord, the Almighty and Majestic, has promised me."