Musannaf Ibn Abi Shayba 19:860

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar Sulayman ibn Hayyan narrated to us, from al-A'mash, from Abu Zabyan, from Usama, who said: The Messenger of Allah (ﷺ) sent us on a military expedition, and we reached the al-Huruqat of Juhayna in the morning. I caught up with a man, and he said: "There is no god but Allah" (La ilaha illa Allah), so I stabbed him. Then, that weighed heavily upon my soul, so I mentioned it to the Prophet (ﷺ). The Messenger of Allah (ﷺ) said: "Did he say '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 open his heart so that you would know whether he said it or not?" He continued to repeat it to me until I wished that I had embraced Islam only on that day.