Musnad Ahmad 1034:145

Muhammad ibn Ubayd narrated to us, Salim al-Muradi narrated to us, from Amr ibn Haram al-Azdi, from Abu Abdullah and Rib'i ibn Hirash, from Hudhayfah, who said: While we were with the Messenger of Allah, may Allah bless him and grant him peace, he said: "I do not know how long I will remain among you, so follow those who come after me"—and he pointed to Abu Bakr and Umar—"and be guided by the guidance of Ammar, and hold fast to the covenant of Ibn Umm Abd."