Sayyiduna Amr bin al-Hamiq al-Khuza'i (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "When a man grants a person safety and then kills him, I am free of the killer, even if the one killed is a disbeliever."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18422