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?”