Usama said, "The Messenger of Allah ﷺ sent us on an expedition against the Huraqah clan of Juhaynah. We attacked the people at dawn and defeated them. A man from the Ansar and I caught up with one of their men, and when we closed in on him, he said, 'There is no god but Allah.' The Ansari held back from him, but I stabbed him with my spear and killed him.When we returned, news of this reached the Prophet ﷺ, and he said, 'Usama, did you kill him after he said, There is no god but Allah?'I replied, 'He was only seeking refuge in those words.'He said again, 'Did you kill him after he said, There is no god but Allah?'He kept repeating it to me until I wished I had not become Muslim before that day."
بعثنا رسول الله ﷺ إلى الحرقة من جهينة، فصبحنا القوم، فهزمناهم، ولحقت أنا ورجل من الأنصار رجلا منهم، فلما غشيناه قال: لا إله إلا الله. فكف عنه الأنصاري، فطعنته برمحي حتى قتلته، قال: فلما قدمنا بلغ ذلك النبي ﷺ، فقال: يا أسامة أقتلته بعد ما قال: لا إله إلا الله؟ قلت: كان متعوذا. قال: فقال: أقتلته بعدما قال لا إله إلا الله؟ قال: فما زال يكررها علي حتى تمنيت أنى لم أكن أسلمت قبل ذلك اليوم.
Bukhari narrated it in Kitab al-Maghazi (4269), and Muslim in Kitab al-Iman (159:96), both of them through Hushaym, who said, our informant was Husayn, who said, our narrator was Abu Dhubyan, who said, I heard Usama bin Zayd bin Haritha narrating, and he said, and then he mentioned it. The wording given is that of Muslim, and the wording of Bukhari is similar.