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Ibn ‘Abbas said : Ubayy b. Ka’b told me that the Messenger of Allah (May peace be upon him) said : Al-khidr saw a youth playing with boys. He took him by his head and uprooted it. Moses then said : Have you slain an innocent person who had slain none.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ مِهْرَانَ الرَّازِيُّ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ عَمْرٍو، عَنْ سَعِيدِ بْنِ جُبَيْرٍ، قَالَ قَالَ ابْنُ عَبَّاسٍ حَدَّثَنِي أُبَىُّ بْنُ كَعْبٍ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ " أَبْصَرَ الْخَضِرُ غُلاَمًا يَلْعَبُ مَعَ الصِّبْيَانِ فَتَنَاوَلَ رَأْسَهُ فَقَلَعَهُ فَقَالَ مُوسَى { أَقَتَلْتَ نَفْسًا زَكِيَّةً } " . الآيَةَ .
sunnah.com