It is narrated from Sayyiduna Mu'adh (may Allah be pleased with him) that he heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever grants a man a promise of safety and then kills him, the Fire becomes obligatory for him, even if the one killed is a disbeliever."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17200