Sahih Muslim, Chapter: Faith (Kitab Al Iman)

that the Messenger of Allah (may peace be upon him) sent us in a raiding party. We raided Huraqat of Juhaina in the morning. I caught hold of a man and he said: There is no god but Allah, I attacked him with a spear. It once occurred to me and I talked about it to the Apostle (may peace be upon him). The Messenger of Allah (may peace be upon him) said: Did he profess" There is no god but Allah," and even then you killed him? I said: Messenger of Allah, he made a profession of it out of the fear of the weapon. He (the Holy Prophet) observed: Did you tear his heart in order to find out whether it had professed or not? And he went on repeating it to me till I wished I had embraced Islam that day. Sa'd said: By Allah, I would never kill any Muslim so long as a person with a heavy belly, i. e., Usama, would not kill. Upon this a person remarked: Did Allah not say this: And fight them until there is no more mischief and religion is wholly for Allah? Sa'd said: We fought so that there should be no mischief, but you and your companions wish to fight so that there should be mischief.