Musnad Ahmad 937:8

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah informed us, from Abdullah ibn Hubayrah, Abu Tamim al-Jayshani informed me, he said: Abu Dharr informed me, he said: I was walking with the Messenger of Allah, may Allah bless him and grant him peace, and he said: "Other than the Dajjal, there is something I fear more for my nation." He said it three times. He said: I said: "O Messenger of Allah, what is this thing, other than the Dajjal, that you fear more for your nation?" He said: "Misguiding leaders."