صحيح مسلم

It is narrated on the authority of Usama b. Zaid: The Messenger of Allah (ﷺ) may peace be upon him) sent us to Huraqat, a tribe of Juhaina. We attacked that tribe early in the morning and defeated them and I and a man from the Ansar caught hold of a person (of the defeated tribe). When we overcame him, he said: There is no god but Allah. At that moment the Ansari spared him, but I attacked him with my spear and killed him. The news had already reached the Apostle (peace be upon him), so when we came back he (the Apostle) said to me: Usama, did you kill him after he had made the profession: There is no god but Allah? I said. Messenger of Allah, he did it only as a shelter. The Prophet (ﷺ) observed: Did you kill him after he had made the profession that there is no god but Allah? He (the Holy Prophet) went on repeating this to me till I wished I had not embraced Islam before that day.