The Messenger of Allah ﷺ said, "Khidr saw a boy playing with other children, so he reached out, took hold of his head, and tore it off. Musa then said, 'Have you killed an innocent soul...'" [Kahf 18:74] the verse.
حدثنا محمد بن مهران الرازي حدثنا سفيان بن عيينة عن عمرو عن سعيد بن جبير قال قال ابن عباس
عن رسول الله ﷺ قال أبصر الخضر غلاما يلعب مع الصبيان فتناول رأسه فقلعه فقال موسى {أقتلت نفسا زكية} الآية
Takhrij (Source Reference): Bukhari narrated it in the Book of Knowledge, in the chapter on what a scholar should say when asked, "Who among the people is most knowledgeable?" leaving that matter entirely to Allah's knowledge, hadith no. 122, and Muslim narrated it in the Book of Virtues, in the chapter on the virtues of Khidr, hadith no. 2380, from the hadith of Sufyan bin Uyayna, with the same content, in a longer form.