Musannaf Ibn Abi Shayba 28:387

Abdullah ibn al-Mubarak narrated to us, he said: Aban ibn Abdullah al-Bajali narrated to us, he said: Amr, the nephew of Ali, narrated to me, from Ali, he said: Ali said: I passed by the Messenger of Allah, may Allah bless him and grant him peace, with camels from the camels of charity. He took a tuft of fur from the back of a camel and said: "Nothing is lawful for me from your spoils except the weight of this, except for the fifth, and it is returned to you."