(It is not for a believer to kill a believer unless (it be) by mistake…) [4:92].
Abu ‘Abd Allah ibn Abi Ishaq informed us> Abu ‘Amr ibn Nujayd> Abu Muslim Ibrahim ibn ‘Abd Allah> Ibn Hajjaj> Hammad> Muhammad ibn Ishaq> ‘Abd al-Rahman ibn al-Qasim> his father who reported that al-Harith ibn Shadid was hard on the Prophet, Allah bless him and give him peace. He was on his way to embrace Islam when he was met by ‘Ayyash ibn Abi Rabi‘ah, and not realising that al-Harith wanted to embrace Islam, he killed him. And so Allah, exalted is He, revealed this verse (It is not for a believer to kill a believer unless (it be) by mistake…).
Al-Kalbi explained this event, saying: “‘Ayyash ibn Abi Rabi‘ah al-Makhzumi had embraced Islam but was fearful of coming in the open about it. He fled to Medina and took refuge in one of its fortresses. His mother was distressed a great deal. She said to her two sons Abu Jahl and al-Harith ibn Hisham, who were his half brothers from his mother’s side: ‘By Allah, I will stay outside in the sun, not eating or drinking, until you bring him back to me’. And they went looking for him, accompanied by al-Harith ibn Zayd ibn Abi Anisah, until they reached Medina. They went to ‘Ayyash in his fortress, and said to him: ‘Come down, your mother has not entered the confines of a house since you left. She has also sworn that she will not eat or drink until you go back to her. By Allah we give you assurance that we will not coerce you to do anything nor will we come between you and your religion’. When they mentioned his mother’s distress and gave him their assurance, he came down to them. They took him out of Medina, tied him up with strings and whipped him a hundred lashes each, and then took him to his mother. His mother said to him: ‘I will not untie your fetters until you disbelieve in that which you have believed’, and he was left fettered in the sun until he gave them some of what they had wanted. Al-Harith ibn Yazid went to him and said: ‘O ‘Ayyash, by Allah, if that which you followed was guidance, you have now left it, and if it was error, you were already in error’. ‘Ayyash got very angry because of what al-Harith said to him and this prompted him to say to him: ‘By Allah, I will kill you if I find you alone’. Afterwards, ‘Ayyash embraced Islam and migrated to the Messenger of Allah, Allah bless him and give him peace, in Medina. Al-Harith ibn Yazid also embraced Islam and migrated to Medina. This happened at a time when ‘Ayyash was not present. As such, he was unaware of al-Harith becoming Muslim. As he was walking outside Quba’, he met al-Harith ibn Yazid. When he saw him, he attacked and killed him. People said to him: ‘What on earth have you done? He had become Muslim!’ ‘Ayyash went back to the Messenger of Allah, Allah bless him and give him peace, and said: ‘O Messenger of Allah, you know what happened between me and al-Harith; I was unaware of his acceptance of Islam and I killed him’. And so Gabriel, peace be upon him, came down with the words of Allah, exalted is He, (It is not for a believer to kill a believer unless (it be) by mistake…)”.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou