Musnad Ahmad 27:179

Abu Mu'awiyah narrated to us, Suhail ibn Abi Salih narrated to us, from his father, from Ibn Umar, who said: "We used to count, while the Messenger of Allah (peace and blessings of Allah be upon him) was alive and his companions were numerous: Abu Bakr, and Umar, and Uthman, then we would remain silent."