Musnad Ahmad 31:833

Ali ibn Ishaq narrated to us, Abdullah informed us, Yunus informed us, from al-Zuhri, Abu Salamah ibn Abd al-Rahman narrated to me, from Abu Sa’id al-Khudri, from the Prophet, peace and blessings of Allah be upon him, who said: "There has never been a successor (khalifah) appointed, except that he had two inner circles (bitanatan): an inner circle that commands him to do good and urges him toward it, and an inner circle that commands him to do evil and urges him toward it. So, the protected one is he whom Allah protects."