Sahih Muslim 96 b
It is narrated on the authority of Usama b. Zaid:
The Messenger of Allah may peace be upon him) sent us to Huraqat, a tribe of Juhaina. We attacked that tribe early in the morning and defeated them and I and a man from the Ansar caught hold of a person (of the defeated tribe). When we overcame him, he said: There is no god but Allah. At that moment the Ansari spared him, but I attacked him with my spear and killed him. The news had already reached the Apostle (peace be upon him), so when we came back he (the Apostle) said to me: Usama, did you kill him after he had made the profession: There is no god but Allah? I said. Messenger of Allah, he did it only as a shelter. The Prophet observed: Did you kill him after he had made the profession that there is no god but Allah? He (the Holy Prophet) went on repeating this to me till I wished I had not embraced Islam before that day.
حَدَّثَنَا يَعْقُوبُ الدَّوْرَقِيُّ، حَدَّثَنَا هُشَيْمٌ، أَخْبَرَنَا حُصَيْنٌ، حَدَّثَنَا أَبُو ظِبْيَانَ، قَالَ سَمِعْتُ أُسَامَةَ بْنَ زَيْدِ بْنِ حَارِثَةَ، يُحَدِّثُ قَالَ بَعَثَنَا رَسُولُ اللَّهِ صلى الله عليه وسلم إِلَى الْحُرَقَةِ مِنْ جُهَيْنَةَ فَصَبَّحْنَا الْقَوْمَ فَهَزَمْنَاهُمْ وَلَحِقْتُ أَنَا وَرَجُلٌ مِنَ الأَنْصَارِ رَجُلاً مِنْهُمْ فَلَمَّا غَشَيْنَاهُ قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ ‏.‏ فَكَفَّ عَنْهُ الأَنْصَارِيُّ وَطَعَنْتُهُ بِرُمْحِي حَتَّى قَتَلْتُهُ ‏.‏ قَالَ فَلَمَّا قَدِمْنَا بَلَغَ ذَلِكَ النَّبِيَّ صلى الله عليه وسلم فَقَالَ لِي ‏"‏ يَا أُسَامَةُ أَقَتَلْتَهُ بَعْدَ مَا قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ ‏"‏ ‏.‏ قَالَ قُلْتُ يَا رَسُولَ اللَّهِ إِنَّمَا كَانَ مُتَعَوِّذًا ‏.‏ قَالَ فَقَالَ ‏"‏ أَقَتَلْتَهُ بَعْدَ مَا قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ ‏"‏ ‏.‏ قَالَ فَمَازَالَ يُكَرِّرُهَا عَلَىَّ حَتَّى تَمَنَّيْتُ أَنِّي لَمْ أَكُنْ أَسْلَمْتُ قَبْلَ ذَلِكَ الْيَوْمِ ‏.‏