Chapter 19: The Book on Sacrifices
Narrated 'Ali: "The Messenger of Allah (ﷺ) prohibited that an animal with a A' dab (stump) horn or ear should be slaughtered as a sacrifice." Qatadah (one of the narrators) said: "So I mentioned this to Sa' eed bin Al-Musayyab and he said: 'The A' dab is that which equals or more than that.'"
Jami at-Tirmidhi 1504