Musannaf Ibn Abi Shayba 36:553

Abu Usama narrated to us, from al-A'mash, who said: Abu Salih narrated to us, he said: Abdullah ibn Salam said: When Uthman was besieged in the house, he said: "Do not kill him, for there remains but little of his appointed time." "By Allah, if you kill him, you will never pray together ever again."