Musannaf Ibn Abi Shayba 30:145

Muhammad ibn Fudayl narrated to us, from ‘Ata’ ibn al-Sa’ib, from Muharib ibn Dithar, from Ibn ‘Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Al-Kawthar is a river in Paradise. Its banks are of gold, and its bed is of jacinth and pearls. Its soil is more fragrant than musk, and its water is sweeter than honey and whiter than snow."