Musnad Ahmad 5:591

Hajjaj narrated to us, Shu'bah narrated to me, from Qatadah, who said: I heard Jurayy ibn Kulayb saying: I heard Ali saying: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade the *'adab* (mutilation/cutting) of the horn and the ear." Qatadah said: So I asked Sa'id ibn al-Musayyib, I said: "What is the *'adab* of the ear?" He said: "When it is half or more than that."