Musannaf Ibn Abi Shayba 28:903

Mu'awiyah ibn Hisham narrated to us, from 'Ammar ibn Ruzayq, from 'Ata' ibn al-Sa'ib, from Abu Yahya, from Ibn 'Abbas regarding [the verse]: "And if he is from a people between whom and you there is a treaty." He said: He is the man who is a confederate (mu'ahid), or his people are people of a treaty. So his blood money is paid to them, and the one who struck him frees a slave.