Musannaf Ibn Abi Shayba 36:97

Abu Khalid al-Ahmar narrated to us, from al-A'mash, from Abu Dhabyan, from Usama, who said: The Messenger of Allah (may Allah bless him and grant him peace) sent us on a military expedition. We reached the al-Huruqat of Juhayna in the morning, and I caught up with a man. He said: "There is no god but Allah (La ilaha illa Allah)," but I stabbed him. That weighed heavily on my soul, so I mentioned it to the Prophet (may Allah bless him and grant him peace). The Prophet (may Allah bless him and grant him peace) said: "He said 'There is no god but Allah,' and you killed him?" I said: "O Messenger of Allah, he only said it out of fear of the weapon." He said: "Did you split his heart open so that you would know whether he said it out of fear of the weapon or not?" He continued to repeat it until I wished that I had embraced Islam only on that day.