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 and whose horn has been half broken."
Thus it is in these two narrations; the first is the sounder of the two and the other is the weaker of the two. And a mawquf report contrary to this regarding the horn has been narrated from Sayyiduna Ali (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19105