It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said:
"Whoever helps to kill a believer, even with half a world, he will meet Allah (SWT) with (the words) written between his eyes, 'He has no hope of the mercy of Allah (SWT)."
English reference: Vol. 3, Book 21, Hadith 2620
Arabic reference: Book 21, Hadith 2718