"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."  

سَمِعْتُ عَلِيًّا يَقُولُ نَهَى رَسُولُ اللَّهِ ﷺ أَنْ يُضَحَّى بِأَعْضَبِ الْقَرْنِ فَذَكَرْتُ ذَلِكَ لِسَعِيدِ بْنِ الْمُسَيَّبِ قَالَ نَعَمْ إِلاَّ عَضَبَ النِّصْفِ وَأَكْثَرَ مِنْ ذَلِكَ