Musannaf Ibn Abi Shayba 18:1024

Abu Bakr narrated to us, he said: Abu Bakr ibn Ayyash narrated to us, from Abu Ishaq, he said: A man came to Umar and said: "I have killed, so is there any repentance for me?" He said: "Yes, so do not despair." Then he recited to him from [Surah] Ha Mim al-Mu'min: "The Forgiver of sin, the Acceptor of repentance, the Severe in punishment."