Musnad Ahmad 32:1536

Ya'qub narrated to us, Abu Uways narrated to us, he said: Ibn Shihab informed me, that his brother informed him, that Anas ibn Malik al-Ansari informed him, that a man asked the Messenger of Allah, may Allah bless him and grant him peace: "What is al-Kawthar?" The Messenger of Allah, may Allah bless him and grant him peace, said: "It is a river that Allah has given me in Paradise, whiter than milk and sweeter than honey. In it are birds whose necks are like the necks of camels." Umar ibn al-Khattab said: "They are indeed soft/delicate, O Messenger of Allah." He said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Those who eat them are softer/more delicate than them."