Surah Al-Kahf 18:74

فَٱنطَلَقَا حَتَّىٰٓ إِذَا لَقِيَا غُلَـٰمًا فَقَتَلَهُۥ قَالَ أَقَتَلْتَ نَفْسًا زَكِيَّةًۢ بِغَيْرِ نَفْسٍ لَّقَدْ جِئْتَ شَيْـًٔا نُّكْرًا

So they set out, until when they met a boy, he [i.e., al-Khiḍr] killed him. [Moses] said, "Have you killed a pure soul for other than [having killed] a soul? You have certainly done a deplorable thing."

Translation: Saheeh International

Tafsir

So they set off after leaving the ship making their way on foot until when they met a boy who had not yet reached puberty playing with other boys among whom his face was the fairest — and he al-Khidr slew him by slitting his throat with a knife while he lay down or by tearing his head off with his hand or by smashing …

Excerpt from Tafsir al-Jalalayn, commentary on 18:74, via Quran.com. The reader has the full commentary and 9 more tafsirs.