Musannaf Ibn Abi Shayba 18:1010

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Harun ibn Sa’d, from Abu al-Duha, he said: I was with Ibn Umar in his tent, and a man asked him about a man who killed a believer intentionally. Ibn Umar recited to him: "And whoever kills a believer intentionally, his recompense is Hell, wherein he will abide eternally," to the end of the verse. Then he said: "Look at whom you have killed."