Musnad Ahmad 1012:53

Mu'awiyah ibn 'Amr narrated to us, Abu Ishaq narrated to us, from 'Abd al-Rahman ibn 'Ayyash, from Sulayman ibn Musa, from Makhul, from Abu Salam, from Abu Umamah, from 'Ubadah ibn al-Samit, who said: The Prophet, peace and blessings of Allah be upon him, took a hair from the side of a camel and said: "O people, it is not lawful for me from what Allah has bestowed upon you as spoils, except for the fifth, and the fifth is returned to you."