Musnad Abu Ya'la al-Mawsili 3:317

Abu Khaythama narrated to us, Sufyan ibn 'Uyayna narrated to us, from 'Abd al-Karim al-Jazari, from Mujahid, from 'Abd al-Rahman ibn Abi Layla, from 'Ali, who said: The Messenger of Allah (may Allah bless him and grant him peace) 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, and he said: "We shall give him from our own [possessions]."