Usama b. Zaid said

God’s Messenger sent us to some people of Juhaina, and I attacked one of them and was about to spear him when he said, “There is no god but God.” I then speared him and killed him, after which I went and told the Prophet. He said, “Did you kill him when he had testified that there is no god but God?” I replied, “Messenger of God, he did that only as a means to escape death.” He asked, “Why did you not split his heart?”* (Bukhari and Muslim.) *He is here rebuked for attributing motives to the man when he could not know his inner motive. Splitting the heart is a figure of speech for examining the inner motives. In the version of Jundub b. ‘Abdallah al-Bajali God’s Messenger is reported as saying several times, “How will you deal with ‘There is no god but God’ when it comes on the day of resurrection?” Muslim transmitted it.