Musnad Ahmad 27:1454

Mu'ammal narrated to us, Hammad—meaning Ibn Zayd—narrated to us, 'Ata' ibn al-Sa'ib narrated to us, he said: Muharib ibn Dithar said to me: "Did you hear Sa'id ibn Jubayr mention anything from Ibn 'Abbas regarding al-Kawthar?" I said: "I heard him say: Ibn 'Abbas said: 'This is the abundant good.'" Muharib said: "Subhan Allah! How rarely does a statement escape Ibn 'Abbas. I heard Ibn 'Umar say: When 'Indeed, We have granted you al-Kawthar' was revealed, the Messenger of Allah (ﷺ) said: 'It is a river in Paradise, its banks are of gold, it flows over pebbles of pearls and rubies, its drink is sweeter than honey, whiter than milk, colder than snow, and more fragrant than the scent of musk.'" He said: "Ibn 'Abbas spoke the truth; by Allah, this is the abundant good."