Musnad Ahmad 32:732

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Qatadah, from Anas, regarding His saying, the Almighty and Majestic: "Indeed, We have granted you al-Kawthar" [Al-Kawthar: 1], that the Prophet, peace and blessings of Allah be upon him, said: "It is a river in Paradise." The Prophet, peace and blessings of Allah be upon him, said: "I saw a river in Paradise, its banks were domes of pearl. I said: 'What is this, O Gabriel?' He said: 'This is al-Kawthar which Allah has granted you.'"