Hujayyah says: We were with Sayyiduna Ali (may Allah be pleased with him) when a man asked: "On behalf of how many people is a cow (sacrificed)?" Sayyiduna Ali (may Allah be pleased with him) said: "On behalf of seven." The man asked about the horn, and Sayyiduna Ali (may Allah be pleased with him) said: "There is no harm in it." The man asked about lameness, and he said: "When the sacrificial animal reaches the place of sacrifice (then it is fine); the Messenger of Allah (peace and blessings be upon him) commanded us to look carefully at the eye and the ear."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19106