Sayyiduna Abu Shurayh al-Khuza'i (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "The most wicked of people before Allah is the one who, in retaliation, kills someone other than the killer, or who, having become a Muslim, demands retaliation from another Muslim for a killing from the time of Jahiliyyah, or who shows his eyes what they have not seen."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15893