Sayyiduna Ali (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) forbade sacrificing an animal whose ear has been half cut off or whose horn has been half broken. Qatadah (may Allah have mercy on him) says: I asked Sa'id bin al-Musayyab (may Allah have mercy on him) about 'adb, and he said: "It means half or more."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19104