Musannaf Ibn Abi Shayba 37:552

Abu Mu'awiyah narrated to us, from al-A'mash, who said: Abu Salih narrated to us, who said: Abdullah ibn Salam said, when Uthman was besieged in the house: "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."