Musannaf Ibn Abi Shayba 18:1279

Abu Bakr narrated to us, he said: Abu Mu’awiyah ibn Hisham narrated to us, he said: ‘Ammar ibn Ruzayq narrated to us, from ‘Ata’ ibn al-Sa’ib, from Abu Yahya, from Ibn ‘Abbas regarding [the verse]: “And if he was from a people who are enemies to you while he is a believer, then the freeing of a believing slave.” He said: A man would come to the Prophet, peace and blessings of Allah be upon him, and embrace Islam, then return to his people while they were polytheists. Then the Muslims would strike him by mistake in a military detachment or an expedition, so the one who struck him would free a slave. Regarding [the verse]: “And if he was from a people between whom and you is a treaty,” he said: He is a man who is a confederate, and his people are people of a treaty, so the blood money is delivered to them, and the one who struck him frees a slave.