Musnad Ahmad 941:4

Hashim ibn Bashir narrated to us, Husayn narrated to us, from Abu Zabyan, who said: I heard Usamah ibn Zayd narrating, he said: The Messenger of Allah (peace and blessings of Allah be upon him) sent us to Al-Huraqat of Juhaynah. He said: So we reached them in the morning and fought them. There was a man among them who, when the people advanced, was one of the fiercest against us, and when they retreated, he was their rearguard. He said: So I and a man from the Ansar cornered him. He said: When we cornered him, he said: "La ilaha illa Allah" (There is no god but Allah). The Ansari refrained from him, but I killed him. This reached the Prophet (peace and blessings of Allah be upon him), so he said: "O Usamah, did you kill him after he said: 'La ilaha illa Allah'?" I said: O Messenger of Allah, he was only seeking refuge from being killed. He repeated it to me until I wished that I had not embraced Islam until that day.