Musannaf Ibn Abi Shayba 18:1074

Abu Bakr narrated to us, he said: Muhammad ibn Bakr narrated to us, from Ibn Jurayj, he said: I heard Sulayman ibn Musa saying: "The 'gathering' (al-ijtima') among us regarding the murdered person is that one man holds him and the other strikes him. So they are both partners in his blood in our view; they are both executed."