A Muslim should not be killed by a non-believer.
بَابُ لَا يُقْتَلُ مُسْلِمٌ بِكَافِرٍ
Sunan ad-Darimi, Book of Blood Money(view book)
Sunan ad-Darimi 2401
Isḥāq > Jarīr > Muṭarrif > al-Shaʿbī > Abū Juḥayfah
"I said to 'Ali: O Commander of the Believers! Do you have anything written that is not in Allah's Book?' He said: 'By the One Who splits the seed and creates the soul, I have not learned from it except what understanding of the Qur'an Allah gives to a man, and what is in this sheet of paper.' I said: 'What is in the paper?' He said: 'It is the 'Aql, the (ransom for) release of captives, and the judgement that no believer is killed for a disbeliever.'" – Sound Chain
أَخْبَرَنَا إِسْحَاقُ أَنْبَأَنَا جَرِيرٌ عَنْ مُطَرِّفٍ عَنْ الشَّعْبِيِّ عَنْ أَبِي جُحَيْفَةَ قَالَ
قُلْتُ لِعَلِيٍّ يَا أَمِيرَ الْمُؤْمِنِينَ هَلْ عَلِمْتَ شَيْئًا مِنَ الْوَحْيِ إِلَّا مَا فِي كِتَابِ اللَّهِ تَعَالَى؟ قَالَ «لَا وَالَّذِي فَلَقَ الْحَبَّةَ وَبَرَأَ النَّسَمَةَ مَا أَعْلَمُهُ إِلَّا فَهْمًا يُعْطِيهِ اللَّهُ الرَّجُلَ فِي الْقُرْآنِ وَمَا فِي الصَّحِيفَةِ» قُلْتُ وَمَا فِي الصَّحِيفَةِ؟ قَالَ «الْعَقْلُ وَفِكَاكُ الْأَسِيرِ وَلَا يُقْتَلُ مُسْلِمٌ بِمُشْرِكٍ» – إسناده صحيح