So they set out, until when they met a boy, al-Khidh r (18:74)

﴿فَانطَلَقَا حَتَّىٰ إِذَا لَقِيَا غُلَامًا فَقَتَلَهُ قَالَ أَقَتَلْتَ نَفْسًا زَكِيَّةً بِغَيْرِ نَفْسٍ لَّقَدْ جِئْتَ شَيْئًا نُّكْرًا﴾
[ الكهف: 74]

18:74 So they set out, until when they met a boy, al-Khidh r killed him. [Moses] said, "Have you killed a pure soul for other than [having killed] a soul? You have certainly done a deplorable thing."

After they disembarked from the ship they set out walking along the seashore, when they saw a young boy who was still not an adult playing with the other boys.
Khidr killed him and Moses said to him: Did you kill a pure soul that is still a minor for no sin? You have really done something evil!!

Then they both proceeded, till they met a boy, he (Khidr) killed him. Musa (Moses) said: "Have you killed an innocent person who had killed none? Verily, you have committed a thing "Nukra" (a great Munkar - prohibited, evil, dreadful thing)!"

phonetic Transliteration

Faintalaqa hatta itha laqiya ghulaman faqatalahu qala aqatalta nafsan zakiyyatan bighayri nafsin laqad jita shayan nukran

Abdullah Yusuf Ali - Translation

Then they proceeded: until, when they met a young man, he slew him. Moses said: "Hast thou slain an innocent person who had slain none? Truly a foul (unheard of) thing hast thou done!"

Safi-ur-Rahman al-Mubarakpuri

Then they both proceeded till they met a boy, and he (Khidr) killed him. Musa said: "Have you killed an innocent person without Nafs Verily, you have committed a thing Nukr!"

Page 301 English transliteration