Sahih Bukhari Volume 5, Book 64, Hadith 4271

حَدَّثَنِي عَمْرُو بْنُ مُحَمَّدٍ، حَدَّثَنَا هُشَيْمٌ، أَخْبَرَنَا حُصَيْنٌ، أَخْبَرَنَا أَبُو ظَبْيَانَ، قَالَ سَمِعْتُ أُسَامَةَ بْنَ زَيْدٍ ـ رضى الله عنهما ـ يَقُولُ بَعَثَنَا رَسُولُ اللَّهِ صلى الله عليه وسلم إِلَى الْحُرَقَةِ، فَصَبَّحْنَا الْقَوْمَ فَهَزَمْنَاهُمْ وَلَحِقْتُ أَنَا وَرَجُلٌ مِنَ الأَنْصَارِ رَجُلاً مِنْهُمْ، فَلَمَّا غَشِينَاهُ قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ‏.‏ فَكَفَّ الأَنْصَارِيُّ، فَطَعَنْتُهُ بِرُمْحِي حَتَّى قَتَلْتُهُ، فَلَمَّا قَدِمْنَا بَلَغَ النَّبِيَّ صلى الله عليه وسلم فَقَالَ ‏"‏ يَا أُسَامَةُ أَقَتَلْتَهُ بَعْدَ مَا قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ ‏"‏ قُلْتُ كَانَ مُتَعَوِّذًا‏.‏ فَمَا زَالَ يُكَرِّرُهَا حَتَّى تَمَنَّيْتُ أَنِّي لَمْ أَكُنْ أَسْلَمْتُ قَبْلَ ذَلِكَ الْيَوْمِ‏.‏

Narrated By Usama bin Zaid : Allah's Apostle sent us towards Al-Huruqa, and in the morning we attacked them and defeated them. I and an Ansari man followed a man from among them and when we took him over, he said, "La ilaha illal-Lah." On hearing that, the Ansari man stopped, but I killed him by stabbing him with my spear. When we returned, the Prophet came to know about that and he said, "O Usama! Did you kill him after he had said "La ilaha ilal-Lah?" I said, "But he said so only to save himself." The Prophet kept on repeating that so often that I wished I had not embraced Islam before that day.