Musnad Abu Ya'la al-Mawsili 115:346

Muhammad ibn Mahdi narrated to us, Abd al-Razzaq narrated to us, Ma'mar informed us, from Qatadah, from Anas, regarding His saying: "Indeed, We have granted you al-Kawthar." The Prophet (may Allah bless him and grant him peace) said: "I saw al-Kawthar, a river in Paradise, its banks are domes of pearl. I said: 'What is this, O Jibril?' He said: 'This is al-Kawthar which Allah has granted you.'"