Musannaf Ibn Abi Shayba 18:524

Abu Bakr narrated to us, he said: Isma'il bin Ibrahim narrated to us, from Khalid al-Hadhdha', from Ibn Sirin, who said: He did not see any harm in a man being killed by two men, that one of them be killed [in retaliation] and he [the guardian of the victim] takes the blood money from the other.