Musannaf Ibn Abi Shayba 27:304

Waki' narrated to us, from Sufyan, from 'Abd al-Malik ibn 'Umayr, from a freed slave of Rib'i ibn Hirash, from Rib'i ibn Hirash, from Hudhayfah, who said: We were sitting with the Prophet, may Allah bless him and grant him peace, and he said: "I do not know how long I will remain among you. Follow the two who come after me," and he pointed to Abu Bakr and 'Umar.