Musannaf Ibn Abi Shayba 28:901

Yahya ibn Sa'id al-Qattan narrated to us, from Sufyan, from Simak, from 'Ikrimah, and from al-Mughirah, from Ibrahim, regarding [the verse]: "And if he is from a people between whom and you there is a treaty." They both said: A man embraces Islam in the Abode of War, then a man kills him; there is no blood money upon him, but there is expiation upon him.