Mishkat al-Masabih 16 Retaliation كتاب القصاص 33

وَعَنْ أَبِي هُرَيْرَةَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَنْ أَعَانَ عَلَى قَتْلِ مُؤْمِنٍ شَطْرَ كَلِمَةٍ لَقِيَ اللَّهَ مَكْتُوبٌ بينَ عينيهِ آيسٌ من رَحْمَة الله» . رَوَاهُ ابْن مَاجَه

Abu Huraira reported God’s Messenger as saying, “If anyone helps in killing a believer to the extent of half a word,* he will meet God with ‘Despairing of God’s mercy’ written on his forehead.” Ibn Majah transmitted it. * This is taken to mean that it is a serious matter even to utter half the word which expresses an intention to kill a believer.