Musnad Ahmad 937:129

Husayn narrated to us, al-Mubarak narrated to us, Abu Na'amah narrated to me, 'Abdullah ibn al-Samit narrated to me, that Abu Dharr said to him: The Messenger of Allah (may Allah bless him and grant him peace) said: "O Abu Dharr, there will surely be leaders," and he mentioned the rest of the hadith.