Musannaf Ibn Abi Shayba 35:31

Hammad ibn Khalid narrated to us, from Aflah, from al-Qasim, from Aisha, that the Prophet (peace and blessings of Allah be upon him) performed *ish'ar* (marking the sacrificial animal). And it was mentioned that Abu Hanifa said: "Marking (*al-ish'ar*) is mutilation (*muthla*)."