Sahih Muslim Book - 1, Hadith - 278

حَدَّثَنَا يَعْقُوبُ الدَّوْرَقِيُّ حَدَّثَنَا هُشَيْمٌ أَخْبَرَنَا حُصَيْنٌ حَدَّثَنَا أَبُو ظَِبْيَانَ قَالَ سَمِعْتُ أُسَامَةَ بْنَ زَيْدِ بْنِ حَارِثَةَ يُحَدِّثُ قَالَ بَعَثَنَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِلَى الْحُرَقَةِ مِنْ جُهَيْنَةَ فَصَبَّحْنَا الْقَوْمَ فَهَزَمْنَاهُمْ وَلَحِقْتُ أَنَا وَرَجُلٌ مِنْ الْأَنْصَارِ رَجُلًا مِنْهُمْ فَلَمَّا غَشِينَاهُ قَالَ لَا إِلَهَ إِلَّا اللَّهُ فَكَفَّ عَنْهُ الْأَنْصَارِيَّ وَطَعَنْتُهُ بِرُمْحِي حَتَّى قَتَلْتُهُ قَالَ فَلَمَّا قَدِمْنَا بَلَغَ ذَلِكَ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالَ لِي يَا أُسَامَةُ أَقَتَلْتَهُ بَعْدَ مَا قَالَ لَا إِلَهَ إِلَّا اللَّهُ قَالَ قُلْتُ يَا رَسُولَ اللَّهِ إِنَّمَا كَانَ مُتَعَوِّذًا قَالَ فَقَالَ أَقَتَلْتَهُ بَعْدَ مَا قَالَ لَا إِلَهَ إِلَّا اللَّهُ قَالَ فَمَا زَالَ يُكَرِّرُهَا عَلَيَّ حَتَّى تَمَنَّيْتُ أَنِّي لَمْ أَكُنْ أَسْلَمْتُ قَبْلَ ذَلِكَ الْيَوْمِ.

Usamah bin Zaid bin Harithah narrated: "The Messenger of Allah (s.a.w) sent us to Al-Huraqah of Juhainah, where we attacked the people in the morning and defeated them. A man from among the Ansar and I caught one of their men, and when we overpowered him, he said: La ilaha illallah. The Ansari left him alone but I stabbed him with my spear and killed him. When we came back, news of that reached the Prophet (s.a.w) and he said to me: 'O Usamah, did you kill him after he said La ilaha illallah?' I said: 'O Messenger of Allah, he was only trying to protect himself.' He said: 'Did you kill him after he said La ilaha illallah?' and he kept repeating it until I wished that I had not become Muslim before that day."