Musannaf Ibn Abi Shayba 18:1276

Abu Bakr narrated to us, he said: Yahya ibn Sa'id al-Qattan narrated to us, from Sufyan, from Simak, from Ikrimah and 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 who embraces Islam in the Abode of War, then a man kills him—there is no blood money (diyah) upon him, but there is upon him the expiation (kaffarah).