Musnad Ahmad 32:1531

Sulayman ibn Dawud al-Hashimi narrated to us, Ibrahim ibn Sa’d informed us, Muhammad ibn Abdullah ibn Muslim, the nephew of Ibn Shihab, narrated to me, from his father, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked about al-Kawthar, and he said: "It is a river that Allah gave me in Paradise. Its soil is musk, its water is whiter than milk and sweeter than honey. Birds with necks like the necks of camels come to it." Abu Bakr said: "O Messenger of Allah, they are indeed soft/delicate." He (the Prophet) said: "Those who eat them are softer/more delicate than them."