Musnad Ahmad 1034:37

Waki’ narrated to us, from Sufyan, from ‘Abd al-Malik ibn ‘Umayr, from a freed slave of Rib’i, from Rib’i, 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 those who come after me," and he pointed to Abu Bakr and ‘Umar. "And hold fast to the covenant of ‘Ammar, and whatever Ibn Mas’ud narrates to you, believe him."