I heard the Messenger of Allah ﷺ say, "Whoever grants a man safety for his life and then kills him will carry the banner of treachery on the Day of Resurrection."
سمعت رسول الله ﷺ يقول: من أمن رجلا على دمه فقتله، فإنه يحمل لواء غدر يوم القيامة.
Ibn Majah (2688) and Ahmad (21946) both narrated it from the hadith of Abdul-Malik bin Umayr, from Rifa bin Shaddad Al-Fityani, who said, "Were it not for a word I heard from Amr bin Al-Hamiq, I would have walked right between the head of Al-Mukhtar and his body." Then he related the hadith. This is the wording of Ibn Majah. As for the wording of Ahmad, "When his lying became clear to me, I resolved, and by Allah, to draw my sword and strike off his neck, until I remembered a hadith that Amr bin Al-Hamiq had told me." Then he related the hadith.The lie that Rifa alluded to is as Ahmad narrated (29947) from Suddi, from Rifa Al-Fityani, who said, "I entered upon Al-Mukhtar, and he threw a cushion down for me, and said, 'Were it not that my brother Jibreel had just risen from this, I would have thrown it to you.'" He said, "So I intended to strike off his neck, but then I remembered a hadith that my brother Amr bin Al-Hamiq had told me, that the Messenger of Allah ﷺ said, 'Whatever believer grants a believer safe conduct regarding his blood and then kills him, I am innocent of the killer.'"Suddi is Ismail bin Abdul-Rahman bin Abu Karima Al-Suddi, whose hadith is hasan. Ibn Hibban also narrated it through his route (5982) without mentioning the story.Al-Mukhtar is Ibn Abu Ubayd Al-Thaqafi, the liar. He was born in the year of the Hijrah and had no companionship with the Prophet nor did he ever see him. He claimed that revelation came to him and that he knew the unseen, until Musab bin Zubayr killed him in Kufa in the year sixty-seven.