Musannaf Ibn Abi Shayba 28:571

Abu Mu'awiyah narrated to us, from al-A'mash, from Abu Zabyan, from Usamah ibn Zayd, who said: The Messenger of Allah (may Allah bless him and grant him peace) sent us to the Huraqat of Juhaynah. He said: We reached the people in the morning and they fled from us. He said: We went out in pursuit of them, and I caught up with a man among them. When I caught up with him, he said: "There is no god but Allah." He said: I thought he only said it out of fear. He said: So I attacked him and killed him. Then, something occurred in my soul regarding his affair, so I mentioned that to the Prophet (may Allah bless him and grant him peace). The Messenger of Allah (may Allah bless him and grant him peace) said to me: "Did he say 'There is no god but Allah,' and then you killed him?" I said: "O Messenger of Allah, he did not say it from his own self; he only said it out of fear of the weapon." He said: "Did he say 'There is no god but Allah,' and then you killed him? Why did you not split open his heart so that you would know whether he said it out of fear of the weapon or not?" Usamah said: He continued to repeat it to me—"Did he say 'There is no god but Allah,' and then you killed him?"—until I wished that I had not embraced Islam until that day.