It was narrated that Rifa'ah bin Shaddad Al-Qitbani said: “Were it not for a word that I heard from 'Amr bin Hamiq Khuza'i, I would have separated the head of Al-Mukhtar from his body. I heard him saying: “The Messenger of Allah (ﷺ) said: 'If a man trusts someone with his life then he kills him, he will carry a banner of treachery on the day of Resurrection.'”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ الْمَلِكِ بْنِ أَبِي الشَّوَارِبِ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ عَبْدِ الْمَلِكِ بْنِ عُمَيْرٍ، عَنْ رِفَاعَةَ بْنِ شَدَّادٍ الْقِتْبَانِيِّ، قَالَ لَوْلاَ كَلِمَةٌ سَمِعْتُهَا مِنْ، عَمْرِو بْنِ الْحَمِقِ الْخُزَاعِيِّ لَمَشَيْتُ فِيمَا بَيْنَ رَأْسِ الْمُخْتَارِ وَجَسَدِهِ سَمِعْتُهُ يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَنْ أَمِنَ رَجُلاً عَلَى دَمِهِ فَقَتَلَهُ فَإِنَّهُ يَحْمِلُ لِوَاءَ غَدْرٍ يَوْمَ الْقِيَامَةِ " .
sunnah.com