Musannaf Ibn Abi Shayba 18:1080

Abu Bakr narrated to us, he said: Muhammad ibn Bakr narrated to us, from Ibn Jurayj, he said: I said to 'Ata', "When does the intellect (diya/blood money) reach the point where the 'aqilah (the paternal relatives) collectively bear it?" He said, "When it reaches one-third." I said, "Yes, I think—and there is no doubt—that he said: And whatever does not reach one-third, then it is upon the man's own people specifically."