Musannaf Ibn Abi Shayba 18:708

Abu Bakr narrated to us, he said: Abda narrated to us, from Hisham ibn Urwa, from his father, that he used to say: The *aqila* (the paternal relatives responsible for blood money) are not held liable for intentional killing, unless they wish to be. And the *aqila* only bear the liability for accidental killing.