Sayyiduna Amr bin al-Hamiq (may Allah be pleased with him) narrates: I heard the Prophet (peace and blessings be upon him) say: "Whoever grants a man safety for his life and then kills him, I am free of the killer, even if the one killed is a disbeliever."
Al-Tabarani recorded it with many chains, and one of them has narrators who are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الديات / 10712