وَعَنْ أُسَامَةَ بْنِ زَيْدٍ رَضِيَ اللَّهُ عَنْهُ قَالَ: بَعَثَنَا رَسُولُ اللَّهِ ﷺ إِلَى أُنَاسٍ مِنْ جُهَيْنَةَ، فَأَتَيْتُ عَلَى رَجُلٍ مِنْهُمْ، فَذَهَبْتُ أَطْعَنُهُ، فَقَالَ: لَا إِلَهَ إِلَّا اللَّهُ، فَطَعَنْتُهُ فَقَتَلْتُهُ، فَجِئْتُ إِلَى النَّبِيِّ ﷺ فَأَخْبَرْتُهُ، فَقَالَ: «أَقَتَلْتَهُ وَقَدْ شَهِدَ أَنْ لَا إِلَهَ إِلَّا اللَّهُ؟» قُلْتُ: يَا رَسُولَ اللَّهِ! إِنَّمَا فَعَلَ ذَلِكَ تَعَوُّذًا، قَالَ: «فَهَلَّا شَقَقْتَ عَنْ قَلْبِهِ؟»
Translation: Usamah ibn Zayd reported that the Messenger of Allah ﷺ sent them to some people of Juhaynah. When Usamah approached a man and was about to strike, the man declared, “There is no deity but Allah.” Usamah nevertheless killed him. On being informed, the Prophet ﷺ asked how he could kill someone who had made that declaration. Usamah said he believed the man spoke only to save himself. The Prophet ﷺ replied, “Why did you not open his heart?”
The question is a severe rebuke: suspicion about a person's inward intention did not justify disregarding his outward declaration.