Musnad Ahmad 2:212

'Abd al-Quddus ibn al-Hajjaj narrated to us, Safwan narrated to us, Abu al-Mukhariq Zuhayr ibn Salim narrated to me that 'Umayr ibn Sa'd al-Ansari—'Umar had appointed him over Homs—... then he mentioned the hadith. 'Umar said—meaning to Ka'b—: "I am asking you about a matter, so do not conceal anything from me." He said: "By Allah, I will not conceal from you anything that I know." He said: "What is the most fearful thing you fear for the nation of Muhammad, may Allah bless him and grant him peace?" He said: "Misguiding leaders." 'Umar said: "You have spoken the truth. That was whispered to me and made known to me by the Messenger of Allah, may Allah bless him and grant him peace."