Mishkat al-Masabih 3450, 3451
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.
وَعَنْ أُسَامَةَ بْنِ زَيْدٍ قَالَ: بَعَثَنَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِلَى أُنَاسٍ مِنْ جُهَيْنَةَ فَأَتَيْتُ عَلَى رَجُلٍ مِنْهُمْ فَذَهَبْتُ أَطْعَنُهُ فَقَالَ: لَا إِلَهَ إِلَّا اللَّهُ فَطَعَنْتُهُ فَقَتَلْتُهُ فَجِئْتُ إِلَى النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَأَخْبَرْتُهُ فَقَالَ: «أقَتلتَه وقدْ شَهِدَ أَنْ لَا إِلَهَ إِلَّا اللَّهُ؟» قُلْتُ: يَا رَسُولَ اللَّهِ إِنَّمَا فَعَلَ ذَلِكَ تَعَوُّذًا قَالَ: «فهَلاَّ شقَقتَ عَن قلبه؟»
وَفِي رِوَايَةِ جُنْدُبِ بْنِ عَبْدِ اللَّهِ الْبَجَلِيِّ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «كَيْفَ تَصْنَعُ بِلَا إِلَهَ إِلَّا اللَّهُ إِذَا جَاءَتْ يَوْمَ الْقِيَامَةِ؟» . قَالَهُ مِرَارًا. رَوَاهُ مُسلم