Sayyidah Aishah (may Allah be pleased with her) narrated that two writings were found in the scabbard of the Messenger of Allah (peace and blessings be upon him): "The most insolent of people is the one who strikes or kills someone other than the one who struck or killed him, and the man who turns to those other than the people who bestowed favour on him. Whoever does that has disbelieved in Allah and His Messenger; neither an obligatory nor a voluntary deed will be accepted from him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15896