´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.'"
Hadith Referenceسنن ترمذي / كتاب الأضاحى عن رسول الله صلى الله عليه وسلم / 1504
´'Ali said:` "Messenger of Allah forbade us from sacrificing an animal with a broken horn." I (the narrator) mentioned that to Sa'eed bin Al_Musayyab and he said: "Yes, m unless half or more of the horn is missing."
Hafiz Muhammad Ameen
In Arabic, the word "a‘dab" has been used. Hazrat Sa‘id ibn al-Musayyib rahimahullah has explained this very word, stating that an animal is not called "a‘dab" due to a slightly broken horn; rather, if half or more than half of the horn is broken, then its sacrifice will be prohibited. Thus, the status of the horn is not like that of the ear. A minor defect in it is excused.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 4382
Narrated Ali ibn Abu Talib: The Prophet ﷺ prohibited to sacrifice an animal with a slit ear and broken horn. Abu Dawud said: The narrator Jurayy (b. Kulaib) is Sadusi, and belongs to Basrah. No one narrated traditions from him except Qatadah.
Shaykh Umar Farooq Saeedi
Benefits and Issues: ➊ One meaning of "Adhba" or "Adhb" is exactly this: that the inner part of the horn is broken. ➋ And the other meaning is that which is found in the following narration: that is, half of the horn is broken or more than half.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 2805