The Messenger of Allah ﷺ sent us on an expedition to al-Huraqah. We attacked the people in the morning and routed them. A man from the Ansar and I caught up with one of their men, and when we overtook him, he said, "There is no god but Allah."The man from the Ansar held back, but I struck him with my spear and killed him.When we returned, the news reached the Prophet ﷺ, and he said, "Usama, did you kill him after he said, 'There is no god but Allah'?"I replied, "He only said it to seek protection."He kept repeating the question until I wished I had not become a Muslim before that day.
بعثنا رسول الله ﷺ إلى الحرقة، فصبحنا القوم فهزمناهم، ولحقت أنا ورجل من الأنصار رجلا منهم، فلما غشيناه، قال: لا إله إلا الله، فكف الأنصاري، فطعنته برمحي حتى قتلته، فلما قدمنا بلغ النبي ﷺ، فقال: يا أسامة أقتلته بعد ما قال: لا إله إلا الله؟ قلت: كان متعوذا. فما زال يكررها حتى تمنيت أني لم أكن أسلمت قبل ذلك اليوم.
Bukhari narrated it in the Book of Military Expeditions (4269), and Muslim in the Book of Faith (96:159), both through the route of Hushaym, who said, Husayn informed us, who said, Abu Dhubyan narrated to us, who said, I heard Usama bin Zayd, and he mentioned it.