"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." (Using translation from Nasāʾī 4377)  

سَمِعْتُ عَلِيًّا يَقُولُ «نَهَى رَسُولُ اللهِ ﷺ أَنْ يُضَحَّى بِأَعْضَبِ الْقَرْنِ» فَذَكَرْتُ ذَلِكَ لِسَعِيدِ بْنِ الْمُسَيَّبِ فَقَالَ نَعَمْ الْأَعْضَبُ النِّصْفُ فَأَكْثَرُ مِنْ ذَلِكَ