Musnad Ahmad 27:906

'Ali ibn Hafs narrated to us, Warqa' informed us, he said: And 'Ata' said, from Muharib ibn Dithar, from Ibn 'Umar, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said to us: "Al-Kawthar is a river in Paradise. Its banks are of gold, and the water flows over pearls. Its water is whiter than milk and sweeter than honey."