Musnad Ahmad 1034:178

Waki’ narrated to us, Sufyan narrated to us, 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, peace and blessings of Allah be upon him, and he said: "I do not know how long I will remain among you, so follow the two who come after me," and he pointed to Abu Bakr and ‘Umar. He said: "And whatever Ibn Mas’ud tells you, believe him."