Musannaf Ibn Abi Shayba 36:554

Abu Usama narrated to us, from Sadaqah ibn Abi Imran, who said: Abu al-Ya'fur narrated to us, from Abu Sa'id, the freed slave of Abdullah ibn Mas'ud, who said: Abdullah ibn Mas'ud said: "By Allah, if you kill Uthman, you will never find a successor for him."