Musnad Ahmad 32:1362

Abu Salamah al-Khuza'i narrated to us, Layth informed us, from Yazid—meaning Ibn al-Had—from 'Abd al-Wahhab ibn Abi Bakr, from 'Abd Allah ibn Muslim, from Ibn Shihab, from Anas, that the Prophet, may Allah bless him and grant him peace, was asked about al-Kawthar, and he said: "It is a river that my Lord gave to me. It is whiter than milk and sweeter than honey, and in it are birds whose necks are like the necks of camels." 'Umar said: "O Messenger of Allah, those are indeed soft birds." He said: "The one who eats them is softer than them, O 'Umar."