Musnad Ahmad 5:537

Waki' narrated to us, he said: Sufyan added, and Abd al-Rahman, from Sufyan, from Abd al-Karim, from Mujahid, from Ibn Abi Layla, from Ali, who said: "The Messenger of Allah, may Allah bless him and grant him peace, commanded me not to give the butcher anything from it [the sacrificial animal] as payment for his butchering."