Narrated Usama bin Zaid bin Haritha:

Allah's Apostle sent us (to fight) against Al-Huraqa (one of the sub-tribes) of Juhaina. We reached those people in the morning and defeated them. A man from the Ansar and I chased one of their men and when we attacked him, he said, "None has the right to be worshipped but Allah." The Ansari refrained from killing him but I stabbed him with my spear till I killed him. When we reached (Medina), this news reached the Prophet. He said to me, "O Usama! You killed him after he had said, 'None has the right to be worshipped but Allah?"' I said, "O Allah's Apostle! He said so in order to save himself." The Prophet ﷺ said, "You killed him after he had said, 'None has the right to be worshipped but Allah." The Prophet kept on repeating that statement till I wished I had not been a Muslim before that day.

حَدَّثَنَا عَمْرُو بْنُ زُرَارَةَ، حَدَّثَنَا هُشَيْمٌ، حَدَّثَنَا حُصَيْنٌ، حَدَّثَنَا أَبُو ظَبْيَانَ، قَالَ سَمِعْتُ أُسَامَةَ بْنَ زَيْدِ بْنِ حَارِثَةَ ـ رضى الله عنهما ـ يُحَدِّثُ قَالَ بَعَثَنَا رَسُولُ اللَّهِ ﷺ إِلَى الْحُرَقَةِ مِنْ جُهَيْنَةَ ـ قَالَ ـ فَصَبَّحْنَا الْقَوْمَ فَهَزَمْنَاهُمْ ـ قَالَ ـ وَلَحِقْتُ أَنَا وَرَجُلٌ مِنَ الأَنْصَارِ رَجُلاً مِنْهُمْ ـ قَالَ ـ فَلَمَّا غَشِينَاهُ قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ ـ قَالَ ـ فَكَفَّ عَنْهُ الأَنْصَارِيُّ، فَطَعَنْتُهُ بِرُمْحِي حَتَّى قَتَلْتُهُ ـ قَالَ ـ فَلَمَّا قَدِمْنَا بَلَغَ ذَلِكَ النَّبِيَّ ﷺ قَالَ فَقَالَ لِي " يَا أُسَامَةُ أَقَتَلْتَهُ بَعْدَ مَا قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ ". قَالَ قُلْتُ يَا رَسُولَ اللَّهِ إِنَّمَا كَانَ مُتَعَوِّذًا. قَالَ " أَقَتَلْتَهُ بَعْدَ أَنْ قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ ". قَالَ فَمَا زَالَ يُكَرِّرُهَا عَلَىَّ حَتَّى تَمَنَّيْتُ أَنِّي لَمْ أَكُنْ أَسْلَمْتُ قَبْلَ ذَلِكَ الْيَوْمِ.