The Prophet ﷺ sent a detachment. I gave a sword to a man from among them. When he came back, he said: Would that you saw us how the Messenger of Allah ﷺ rebuked us, saying: When I sent out a man who does not fulfil my command, are you unable to appoint in his place one who will fulfil my command. (Using translation from Abū Dāʾūd 2627)   

بَعَثَ رَسُولُ اللهِ ﷺ سَرِيَّةً فَسَلَّحْتُ رَجُلًا سَيْفًا قَالَ فَلَمَّا رَجَعَ قَالَ مَا رَأَيْتُ مِثْلَ مَا لَامَنَا رَسُولُ اللهِ ﷺ قَالَ أَعَجَزْتُمْ إِذْ بَعَثْتُ رَجُلًا فَلَمْ يَمْضِ لِأَمْرِي أَنْ تَجْعَلُوا مَكَانَهُ مَنْ يَمْضِي لِأَمْرِي؟  

[Machine] While the Messenger of Allah ﷺ was giving a sermon, someone said, "O Messenger of Allah, I swear by Allah that the one who said what he said did not say it except seeking protection from being killed." So the Messenger of Allah ﷺ mentioned his story and approached him. The trace of anger was visible on his face, and then he said, "Verily, Allah has forbidden me to kill a believer." He repeated this three times.  

بَيْنَمَا رَسُولُ اللهِ ﷺ يَخْطُبُ إِذْ قَالَ الْقَائِلُ يَا رَسُولَ اللهِ وَاللهِ مَا قَالَ الَّذِي قَالَ إِلَّا تَعَوُّذًا مِنَ الْقَتْلِ فَذَكَرَ قِصَّتَهُ فَأَقْبَلَ عَلَيْهِ رَسُولُ اللهِ ﷺ تُعْرَفُ الْمَسَاءَةُ فِي وَجْهِهِ ثُمَّ قَالَ إِنَّ اللهَ ﷻ أَبَى عَلَيَّ مَنْ قَتَلَ مُؤْمِنًا قَالَهَا ثَلَاثَ مَرَّاتٍ  

[Machine] Once, a group of people who were on a secret mission for the Messenger of Allah ﷺ approached a well in the morning. A man from the people of the well appeared and one of the Muslims apprehended him. The man said, "I am a Muslim," but he was killed. When they returned, they informed the Prophet ﷺ about what had happened. The Messenger of Allah ﷺ stood up to address the people, praised Allah and glorified Him, then said, "What is wrong with a Muslim killing another man while the latter is saying, 'I am a Muslim'? The man said this as a means of seeking refuge." Upon hearing this, the face of the Messenger of Allah ﷺ turned away, and he stretched out his right hand and said, "I swear by Allah, the one who kills a Muslim will never enter Paradise." He repeated this statement three times.  

أَنَّ سَرِيَّةً لِرَسُولِ اللهِ ﷺ غَشُوا أَهْلَ مَاءٍ صُبْحًا فَبَرَزَ رَجُلٌ مِنْ أَهْلِ الْمَاءِ فَحَمَلَ عَلَيْهِ رَجُلٌ مِنَ الْمُسْلِمِينَ فَقَالَ إِنِّي مُسْلِمٌ فَقَتَلَهُ فَلَمَّا قَدِمُوا أَخْبَرُوا النَّبِيَّ ﷺ بِذَلِكَ فَقَامَ رَسُولُ اللهِ ﷺ خَطِيبًا فَحَمِدَ اللهَ وَأَثْنَى عَلَيْهِ ثُمَّ قَالَ أَمَّا بَعْدُ فَمَا بَالُ الْمُسْلِمِ يَقْتُلُ الرَّجُلَ وَهُوَ يَقُولُ إِنِّي مُسْلِمٌ فَقَالَ الرَّجُلُ إِنَّمَا قَالَهَا مُتَعَوِّذًا فَصَرَفَ رَسُولُ اللهِ ﷺ وَجْهَهُ وَمَدَّ يَدَهُ الْيُمْنَى فَقَالَ أَبَى الله عَلَيَّ مَنْ قَتَلَ مُسْلِمًا ثَلَاثَ مَرَّاتٍ