Musnad Abu Ya'la al-Mawsili 3:38

Ubayd Allah narrated to us, Sufyan narrated to us, from Abd al-Karim al-Jazari, from Mujahid, from Ibn Abi Layla, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) commanded me to stand over his sacrificial animals (budn) and to distribute their skins and their coverings. And he commanded me not to give the butcher anything from them; we give him from our own [possessions].