Musnad Ahmad 941:60

Ya'la narrated to us, Al-A'mash narrated to us, from Abu Dhabyan, Usamah ibn Zayd narrated to us, he said: The Messenger of Allah (peace and blessings of Allah be upon him) sent us on an expedition to Al-Huruqat. They became aware of us and fled. We caught up with a man, and when we surrounded him, he said: "La ilaha illa Allah" (There is no god but Allah). We struck him until we killed him. Something regarding that occurred in my soul, so I mentioned it to the Messenger of Allah (peace and blessings of Allah be upon him). He said: "Who will defend you against 'La ilaha illa Allah' on the Day of Resurrection?" I said: "O Messenger of Allah, he only said it out of fear of the weapon and being killed." He said: "Did you split open his heart so that you would know whether he said it for that reason or not? Who will defend you against 'La ilaha illa Allah' on the Day of Resurrection?" He continued to say that until I wished that I had not embraced Islam until that very day.